I cannot speak for Dennett, but I, personally, find Materialism and Darwinism appealing, because, despite their obvious flaws, they also, quite obviously, describe and explain the manifested world more accurately than other attempts at the same.

Furthermore, when you look at atheism you will realize that all atheists without exception argue against religious concepts of god(s), i.e. god(s) as they are interpreted by people associated with respective religious or spiritual institutions. In that sense I'm an atheist, too.

So, the question to me, in this case, is not so much why Materialism, Darwinism, or Atheism are appealing, but why people you allude to vehemently refuse even an innocent attempt at understanding what ancients meant when they talked about e.g. God or spirits.

And the answer to that is IMO simple — bias.

To make the long story short: religious crowds tried (and continue to try) to decipher the "mystery" but they generally fail and consistently continue to do so for millennia. Mostly because the livelihood of priests depends on "blind" masses. After all, who needs a priest when they are with God 24/7. And anyone can observe how much social power this puts in the hands of priests.

Enter the atheist armed with science.

He dispels all religious dogma with utmost ease and reveals how untenable all those interpretations are. So far so good, but when he victoriously proclaims God to be dead, he does not realize that he didn't kill the God, but all he has done was killing an image of God, an idol. Thus he assumes that he also refuted the source material that was being interpreted and doesn't bother to scrutinize it himself (or scrutinizes it through the lens of aforementioned assumption). Kind of Catch 22 situation.

Thus where an average religious believer lives in a mental fantasy world, an average depressed atheist is merely inconsequent[1]. However the end result is the same — ignorance.

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[1] One of the greatest, if not the greatest, “sins” in academia, thus also very hard obstacle to acknowledge and overcome.