The reason why most atheist philosophers don't do philosophy of religion is a lot simpler than most of the explanations I have been seeing. A theistic philosopher is developing an understand of what they do believe, so they are going to devote their attention to it. An atheist philosopher is engaged in explaining why he or she doesn't believe something. The truth about reality, as the atheist sees it, has to be developed in some other way. Denial of the existence of God doesn't tell us anything about what does exist, it only indicates what does not exist. Atheists can be strongly naturalistic like Dennett, or very non-naturalistic like Nagel. In general people spend more time on what they think is true than on what they think isn't, unless they think there's a mind virus out there they think they can get rid of. But most atheist philosophers that I have encountered don't think this. They may not think someone like Plantinga is right, but they are happy to see his point of view competently represented. Here's an excellent example (though I realize many atheists thinks Nagel is a traitor).