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?
I believe that it isn't considered as religion because religion is known to be our guide closer to God and it makes our faith stronger. Athiesm is actually being against religion because they don't believe in the exist. They are known to be anti religion so they are another group but they aren't considered as religion because it actually contradicts so in differs from religion.