They just don't get it...
Discussions on atheism and religion will frequently bring theists around to the idea that atheism is a religion. Atheist Camel has put it into his version of a twisted Christian equation for us: "Non-belief = religion".
Nothing could be less true, of course, and that's part of the reason why atheists turn to pretending belief in some rather ridiculous notions of gods like the flying spaghetti monster in order to mock religion.
Belief requires "a something" in which to place ones faith. Atheism doesn't offer any "somethings". Atheism is quite simply the notion that "gods" are fantasy. That's it.
If atheists desire to believe in "a something" (and there's no reason to think that we do) than "something" must first present itself with either undeniable evidence of its actual being or by some other means, show itself clearly through demonstrable cause and effect existence.
The closest things I can think of to any sort of god-figures might be gravity or infinite space or absolute oblivion or the like. And gosh, since those things already have names, why call them "gods".