Because real atheists simply don't believe in God. End of story.
"Atheists" are actually anti-theists, at least the ones we're discussing. That is to say they actually do believe in God but they hate God to such an extent they won't even admit to themselves how much they do care. They are in classical denial, rather than simply being agnostic or atheist.
I once read that Hell is not actually fire but a sort of passion that can't be consumed. Folks, this is exactly what being an anti-theist is like. They can't bring themselves to love a God who as they see allows evil (usually they talk about big evil, but privately think of small personal evil, like allowing their mom to die of cancer), but they also can't stop talking about this God.
I'll probably get warned for hate speech (why I don't do forums much anymore), but this is not my intention at all. For you see, I don't hate this sort of person. I understand them completely. I've known frustration with a God that won't always give us what we want. I've been there. And I've seen this same behavior in relationships. You tell yourself you're over this person, you go on a date with a new one, then start talking the whole date about the person. That's someone still in love. And that's the point.
I'm a pantheist (or, more precisely, a panentheist, since most of the former don't believe in a distinct God but rather the universe). To me, God is my therapist, my mother, my best friend, my lover. To say God doesn't exist, and then to talk on and on about God, when you could have just said "I don't feel like God exists for me" feels too much like complaining about your dentist, while at the same time thinking about the next appointment .