because an atheist don't believe in the god until proven otherwise imo, so they put they put the onus on religion to prove the existence of god or any supernatural entity. classifying atheism into other religion would make a collection of people who are doubtful & doubt cannot equate with faith so the combination would be a complete absurdity. folklores and mythological stories should always be taken with a pinch of salt. also the debate is quite interesting if religious people bring strong cases in defence.