It's because religion provides a very good story. We see films for entertainment. Therefore, as better story will usually better entertainment (not to mention public bias

), the religious films will be either more interesting or sell better regardless. So we make more about religious truth than the lack of it. I guess it goes something like that. I guess all films that fail to mention god at all could be seen as atheist, but they could just as easily be universally religious, or religious in a deeply private and/or personal sense so no one talks about it much.
I wonder what a good film -about- atheism, not featuring atheism like
Contact or something but about it, could be like. Maybe someone who really
relies on their beliefs has a crisis of faith, realises God isn't real and slowly goes insane as they start to imagine miracles etc. Everyone around them assures them they aren't happening and the character is torn between believing her friends (who would have to clearly be right for it to be an atheist film) or her own senses. I can't see it ending well in any way, though.

Anyway. That's my look at it. If anyone feels a burning need to know; I'm atheist.