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?
It is a matter of understanding the word "religion"
Maybe because religion is always define as a practice of belief with superstitious understanding, and also atheists don't have a weekly activity and discuss many things about their philosophy unlike religious people.
So it doesn't matter if they are a religion or not a big deal. The big deal is if you are belong to religion be active on it, do your duty to keep yourself from being hypocrite.