First, I don't think it's fair to say that atheists commit the genetic fallacy for the simple reason that atheists need not use an argument of the type you are suggesting, nor do they feel the need to.
It is true that very often atheists claim that their ability to explain theistic beliefs offers them a reason for rejecting theistic beliefs as false. Now if you had an explanation for theistic belief that showed that most human beings would believe in God regardless of the state of the evidence, that might be of some use argumentatively if it also showed that this was not the case for unbelievers. But do we have this? When someone like Freud comes up with a possible theory explaining the psychology of religious belief, that doesn't do a whole lot. We have a clearly true explanation for how we got our information about Hobbits. Theism is just a much tougher case.
So we get the argument "Humans fear mortality so much that they will invent any theory, however unreasonable, to escape the conclusion that they are headed for extinction." And now we're going to need the hard evidence, suggesting that this psychological theory is true. I can tell you that in my own case, wanting to believe in God made it more difficult to believe in God, not easier to believe in God. There are possible explanations for why I might have misevaluated the evidence in favor of theism, but these possible explanations don't do much to undermine my beliefs.
This is especially so since it's easy to come up with psychological explanations for atheism. Freud had a good one: the Oedipus complex. Or just the natural human desire to be the supreme beings, to have no one in existence who can judge my actions and tell me definitively that I am wrong, that I am a sinner. It seems to me that I can tell the atheist "OK, you take your psychoanalytic arguments off the table, and I'll take mine."
So while an industrial strength explanation for religious belief that really shows that people, including those like me who think they have good reasons for believing in God, would believe whether there were good reasons or not, might be damaging to religious belief, I don't think we are anywhere near having that kind of an explanation.