Real Atheists, not the ones who like to call themselves atheists for fun, always has a good proof-based explanation for it, I believe Religion makes them feel like being inside a Petri dish, or they notice that religion is like walking with a cloth on the eyes. But even whith all the disadvantages a religion may have, I think mankind would be chaos without having something to believe...
You'd be amazed at the peace of mind you get when you realize you don't have to believe in anything. You can just exist.
Then when calamity hits this kind of person, and he is forced to believe something, he literally goes crazy, because he isn't familiar with believing. Then he commits suicide and doesn't exist any longer.
EDIT: Of course, that is nothing compared with how freaked out he will be in the resurrection when he is forced to believe God exists.