The reason is that the modern project of the philosophy of mind is Cartesian through and through. Daniel Dennett, despite his protests, is about as Cartesian as he could possibly be. The modern philosophy of mind, in fact, never seriously challenges Descartes. Philosophers attack Descartes' dualistic conclusions, but authoritatively assert the foundational elements of Cartesian philosophy that drove Descartes to dualism. These same foundational elements set the modern philosophy of mind on a path to dualism, and like a man trying to go to Los Angeles after setting his GPS for New York, modern philosophers spend their efforts trying to avoid the conclusions their first principles dictate they must eventually accept.
In Ch. 2 of Consciousness Explained, for example, Daniel Dennett gives reasons why dualism will never work as a philosophy. The dualist won't be able to explain how the immaterial mind can interact with a physical body anymore than Caspar the Ghost can explain why he can move through walls yet hold up a house when it is about to fall down. Dennett's criticisms hit the mark, but it doesn't address the reasons Descartes became a dualist in the first place. Cartesian dualism follows directly from the methodical first principle of Cartesian philosophy:
"I thought that I should take a course precisely contrary, and reject as absolutely false anything of which I could have the least doubt, in order to see whether anything would be left after this procedure which could be called wholly certain. Thus, as our senses deceive us at times, I was ready to suppose that nothing was at all the way our senses represented them to be... But I soon noticed that while I thus wished to think everything false, it was necessarily true that I who thought so was something. Since this truth, I think, therefore I am, was so firm and assured that all the most extravagant suppositions of the skeptics were unable to shake it, I judged that I could safely accept it as the first principle of the philosophy I was seeking.I then examined closely what I was, and saw that I could imagine that I had no body, and that there was no world nor any place that I occupied, but that I could not imagine for a moment that I did not exist.... therefore I concluded that I was a thing or substance whose whole essence or nature was only to think, and which, to exist, has no need of space nor of any material thing or body. Thus it followed that this ego, by which I am what I am, is entirely distinct from the body and is easier to know than the latter, and that even if the body were not, the soul would not cease to be all that it now is."
The essence of Cartesianism is in the first paragraph, not the second, which is merely a conclusion from Cartesian first principles. Those first principles are 1) The assertion of method as foundational to true philosophy, and 2) The selection of radical doubt as the method of choice. We have become so used to the Cartesian first principles that we tend to see past them and take them as self-evident first principles of thought itself. But they are not self-evident at all; at least they were not for Descartes. He spent the first part of the Discourse on Method justifying his beginning philosophy in method and radical doubt (which, once that doubt is asserted, makes one wonder about the cognitive status of the first part of the Discourse, since it is asserted prior to and without the benefit of the method.) To the extent that we see the basic task of the philosopher as to "doubt things", or think that we need special training in order to philosophize, we have adopted the Cartesian approach to philosophy. For "special training" is nothing other than education in technical method, and that doubt should be a first principle of philosophy is itself open to philosophical doubt.
Daniel Dennett begins Consciousness Explained by introducing the "brain in a vat" thought experiment, which he admits is a modern version of Descartes' Evil Demon. You've no doubt heard this before: How do you know that you are not merely a brain in a vat, with electrodes hooked up to your neurons, making you think reality is something completely different than it truly is? It's easy to see that this is another way of posing the possibility that "nothing at all was the way our senses represented them to be." Dennett rapidly concludes that you are not a brain in a vat, arguing from scientific considerations of the difficulty of pulling off something like the brain in the vat hoax on a real brain. Of course, Dennett's destruction of the vat doesn't really work, because his argument depends on his knowledge of the way the real world really works; in other words, his argument starts with his brain outside the vat in the real world. Like David Copperfield, he only appears to have gotten himself out of the vat. He was already out the whole time.
But that is beside the point. It doesn't matter whether the brain in the vat experiment pans out. The basic Cartesian principle is that the radical falsity of experience is a possibility that must be addressed and overcome at the very outset of philosophy; Descartes himself overcomes this radical doubt, although in a way different than Dennett. The point is that any philosophy that feels it must start with the overcoming of radical doubt is starting on the Cartesian railroad.
The second fundamental principle of Cartesian philosophy is that philosophy can only be conducted in the light of method. The point here is to undermine "folk philosophy" or the naive trading of opinions that was supposed to be characteristic of traditional philosophy. Instead of lolling around the agora engaging in idle conversation, the modern philosopher rolls up his sleeves and gets results. In Descartes' words, the ancient philosopher only argued the truth; the modern philosopher discovers the truth.
Descartes' method of choice was that of applied universal doubt, but the selection of the particular method is not so important as the decision that philosophy itself can only begin with method. The latter principle is the distinguishing one of Cartesian philosophy. Since Descartes' time, philosophers have tried various experimental combinations of doubt and method; criticizing each other's doubt as not being real doubt, or each other's method as being poorly applied or wrongly selected, but the presumption that philosophy must begin with some form of doubt and method has been more or less tacitly assumed throughout the history of modern philosophy.
The contemporary philosophy of mind generally begins in straightforward Cartesian terms with the assertion of method, in this case scientific method. There is nothing wrong with referring to scientific results in philosophy, of course, but what distinguishes the approach as Cartesian is that science is brought in through authoritative assertion rather than argument. This is the way John Searle does it in Mind, A Brief Introduction:
"The view implicit in this book, which I know want to make explicit, is that science does not name an ontological domain; it names rather a set of methods for finding out about anything at all that admits of systematic investigation... There is no such thing as the scientific world. There is, rather, just the world, and what we are trying to do is describe how it works and describe our situation in it. As far as we know, its most fundamental principles are given by atomic physics and, for that little corner of it that most concerns us, evolutionary biology. The two basic principles on which any such investigation as the one I have been engaging in depends on are, first, the notion that the most fundamental entities in reality are those described by atomic physics; and, second, that we, as biological beasts, are the products of long periods of evolution, perhaps as long as five billion years."
So if we are able to discover anything about the mind systematically, it must be through the methods of science, which has already established atomic physics and Darwinian evolution through the application of method. The interesting Cartesian question is: What is the relationship of the subsequent philosophy of mind to the mind that authoritatively established method and its results in the first place? Can the philosophy of mind call into question the mind that established its foundation in method? It is the same question that can be asked of Descartes, and it brings to light the inevitable tendency of Cartesian thought towards dualism.
In his Discourse on Method, Descartes proclaims his method in Part II after a preamble in both Parts I and the beginning of Part II. The method is supposed to cast all prior notions into doubt so as to find the one indubitable starting point of philosophy. But if the method does this, what happens to the cognitive status of the preamble? Is it not cast into doubt as well? The preamble consists of Descartes' reasons for abandoning the traditional approach to philosophy and inventing a new approach. It involves his views on the historical futility of philosophy and the uselessness of what he learned in school. But if we are to doubt all prior notions, should we not also doubt the uselessness of traditional philosophy and the worthlessness of what Descartes learned in school? Should not Descartes doubt that he ever was in school, or that he ever learned philosophy? Such doubt would, of course, undermine Descartes' justification for his revolution in philosophy. It would bring his project to a standstill. In fact, Descartes does not really doubt everything; he doesn't doubt his own appreciation of the history of philosophy or his confidence in establishing a radically new basis for philosophy. His assertion of the Method merely hides his earlier conclusions, which were not established by the method but are nonetheless beyond all doubt. The mind that established those conclusions, and that authorized and created the Method, is itself also hidden from view. But although it is hidden, it still lurks in the background, and will never go away because it is more certain than the Method itself. This is the ghost that reasserts itself in the form of Cartesian dualism; the Cartesian ghost is the true knower who established and underwrites the Method through which all other beings are granted existence. The Cartesian world is a world of beings who are granted existence through method; but, as Descartes realized, the Thinking Being who conducts the Method is not itself granted existence through Method, for it must already be for the Method to happen at all. So the thinking being is not a body or extensive being in the world like all others, it is an immaterial being transcending the world entirely. Thus we arrive at dualism.
The contemporary philosopher of mind follows the same path as Descartes, with the method of empirical science substituted for the method of empirical doubt. But the result is the same. For John Searle, the true "fundamental entities" populating the world are those established by the methods of atomic physics. What of the scientific mind that creates, establishes, and conducts atomic science, and proclaims in its name the true fundamental entities? This Thinking Being is clearly less dubitable than the atomic particles it proclaims, and it is also beyond the reach of the philosophy of mind, for the philosophy of mind starts with the scientific mind behind it as the authoritative voice of method. But although hidden, the scientific mind is still there, and haunts the contemporary philosophy of mind in the form of the Cartesian Ghost. Daniel Dennett won't find the Cartesian Ghost in his Cartesian Theater; he'll find him in the scientist who establishes the scientific results with which Dennett starts the philosophy of mind.
The only way to exorcise the Cartesian Ghost is to stop repeating the spell that calls him forth from the grave: The insistence on doubt and method as first principles of philosophy.