because atheist was a word before it was used (as you want to use it, i suppose) to define a group. You can't change the meaning of a word because you don't fit into it's definition, you pick another word for your position.
Although the gay movement did manage to change the meaning of gay I suppose. But with atheism I don't think you'll be able to, realistically speaking.
Although the gay movement did manage to change the meaning of gay I suppose. But with atheism I don't think you'll be able to, realistically speaking.