Who says there isn't?

It is true that Atheismis is not an organized and structured community, with dogmas or hierarchy. There's no need to be part of any collective in order to be an atheist. But atheism is growing. Many atheist feel the need to gather with self-minded people to share thoughts and express our opinions. There are atheist and secular communities, movements and associations almost in every big civilized city. And many other international ones, like The Out Campaign, the Atheist Alliance International, and many more. I'm a member of some of them.

You can find some of them here: http://en.wikipedia.org/wiki/List_of_secularist_organizations