Atheists are (generally) rationalists. They draw their conclusion that there probably isn't a God¹ because there is no evidence of one. So they move through life with the assumption there is no God, but should evidence come to light supporting His existence, they'll re-examine their position. In other words, "show me the money!"

Then you get the "strong" atheists who are less than rational and move towards fundamentalist thinking. Their position is that they know there is no God, which is a real trick since they're claiming absolute knowledge about something. In other words, even in the face of evidence supporting the existence of (a) God, they intend to argue that it's a trick, or misleading or what-have-you. More on strong and weak atheism here.

_{¹ "God" not only covers the Big Three patriarchal, transcendent Gods, but also various other gods, including Jeff, God of Biscuits.}