I think they like to reinforce a feeling of superiority over people of faith. It must be so nice to be so intelligent and have the universe all figured out by the time you're in your mid-twenties.
Think we have everything figured out? That sounds like theists who are sure that a magic sky man magic'd everything into existence.
Most atheists say they don't know how everything got started. We're waiting for science to find out more and more about that. You guys are the ones who think you've got it all figured out, because of certain beliefs passed down from your parents to you from an old book of fairy tales written by desert savages who thought the Earth was flat.