Probably something parallel to the reason that, if there is a god who does want to be believed in, he apparently created people who feel inclined to write things like "The God Delusion".

(One possibility: Satan planted the Bible, the Qur'an, etc. in rebellion against God's desire to not be believed in. Ever since then, God's been doing desperate damage control by watching over torture, rape, and genocide, and not doing anything, but to little avail — people go right on believing in him, because Satan's memes are just too infectious and powerful.)