Why are religious forums filled with athiests trying to convince people not to believe in God, first because their mission is practically impossible because few believers will change to athiesm,
I have no doubt that a lot of what you describe as atheists trying to change believers to atheism is actually atheists just criticizing aspects of different religions and beliefs about god(s) that they personally disagree with or find fault in (much the same way as theists do with concepts they disagree with or find fault in).
That said, what you described does exist, whether when it's done in a good spirit and respectfully or not. The mission in that case isn't really in any sense impossible, and saying that few believers will change to atheism is basically just an avoidance of reality, as far as i see it. Not that a lot of people would change their beliefs on account of frequenting a forum, but it happens (in all directions), and it can be just a step or a portion of a journey to a change rather than the sole reason responsible.
Now, all the above said, at least when it comes to this forum, what you described would actually most likely constitute a rule violation.
what's the point when they could be on other forums chatting about things they do believe in?
You're misunderstanding the premise. Assuming you're at least partially talking about this forum, it isn't a forum for promoting (or exclusive discussion of) certain brands of theism or theism in general. It's a forum for discussion and debate of different world views, including atheism itself as a world view. There's specifically a DIR called "Non-Theistic/Non-Religious Beliefs DIR", even, and a "Non-theism" sub-forum under Theological concepts. In addition, there are atheists who embrace religious beliefs, and most atheists are from a religious background.
It's always been a mystery to me why athiests spend so much time and energy on religious forums when they could be doing other things like ........
Whether or not someone believes in god(s) has little to do with the fact that religion and specifically belief in god are serious cultural forces in most places around the world today. They're relevant, rich and interesting subjects to discuss and more importantly, they relate to social forces that can affect people's lives (atheist or not) in manners ranging from being proselytized to, to being under threat of facing consequences for holding a specific belief.
