Why do you feel the need to ask me that question? Couldn't you go out and play in the middle of a busy street instead?
The answer is: Because I don't believe in the existence of deities. That's the usual answer.
But that's not what all these passive-aggressive little rodents are really asking. What they really want to know is why don't I just keep it to myself, please? And the answer to THAT is: Why not talk about it? The answer is: Screw you.
I identify as an atheist because I can. Just doing my little bit to try to keep theocracy from returning.