Yet they almost completely ignore Islam. William Kilpatrick has been doing a ton of writing on the subject of Islam for a long time. From his latest:
As I say, these assertions about the authenticity of the revelation appear over and over. Far more space is allotted to vouching for the genuine nature of the revelation than to telling what the revelation is. But what sort of author feels compelled to tell us ad nauseum that his word is not a human invention? It’s not likely that the Author of all Creation would be so insecure about what he had written. On the other hand, a man who had invented it all himself would have good reason to be defensive. Muhammad, however, also realized that the best defense is a good offense. Thus, as the Koran repeatedly reminds its readers, the surest path to hell is to doubt “Our revelations.”
In insisting that the Koran is the verbatim word of God, Muslims are stuck with the task of defending a second-rate literary production as though it were Shakespeare, Homer, and Dante all rolled into one. If they have been largely successful in so defending it, it is because not many want to challenge them on the point.
You wonder why that is. Why don't atheist have the inclination to attack Islam? One reason is fear. Certainly Bart Ehrmans has said vary clearly that the reason he does not attack the Koran the way he does the bible is because he is afraid of Muslim violence. I can see that for a big name scholar who likes to make shocking statements. I don't see it near as much for the typical atheist who does not use his real name and is really not likely to get that much attention.