My view on the matter is that there are those individual atheists, or atheistic movements, that do have a lot in common with various religions; and because they are loud, and vocal, and make a good try at being missionaries in their own way, they are also the groups of atheists that a lot of people will run into first, despite quite possibly being in the minority; and thus, despite being quite possibly a minority, there exist quite a lot of people who have only, ever met that sort of atheist; that is to say, in their minds, that is the pattern which the word "atheist" matches to.