As pointed out by others, Dawkins does consider the evolutionary mechanisms behind evolution. In The God Delusion he offers additional possible mechanisms to the one you mention. But Dawkins' goal is not to understand the mechanism by which religion came about. Rather, it is to show people that religion is both wrong and harmful.

You see, even if you find the spot in the brain that causes people to believe in God (if such a thing exists), it will not convince anyone that there is no God. The believers will simply say that God put it there. If you want to convince, logically, that there is no God, you have to do so at a philosophical level like Dawkins does. Having said that, I think it that there are more effective ways to achieve this goal than to ram people with the truth head-on like Dawkins does.