Since atheists have a common belief system and they are pretty organized why they do not consider themselves as a part of a another religion which does not have a god?
Because not playing sport is not a sport.
If you are trying to say that atheists don't believe anything at all, you are kinda dishonoring them.
Atheists are people. They have the ability to believe things, and not believe other things, just like all the other people. What are you saying? That they are some kind of inferior creature, like a stupid, dumb animal, by saying that they don't believe in anything at all?
It depends how you define belief. Blind belief is a religious must. You can however believe in things when there is enough evidence even though it's not 100% proved. Religious folks are told god exists without any proof and they simply believe it which is what makes them religious.