That's a rather naive, if popular, view of religion.

Probably made even more popular by the fact that in the US any kind of fringe BS can pass as a religion, from Scientology to what have you.

And by fringe I don't mean less true in the "god exists" way (well, he doesn't exist, at least not in any religious guise), but less historically established and having served an important societal role.

Because religions were an early cultural framework for formulation of a code of ethics, building ideas about society, life, etc. That we today deem some of those ideas bad doesn't mean they were bad when they formed (or even now), or that they didn't play an important (positive I mean) role in society's development.

(The inverse idea is usually based on two naive beliefs: religious people were just dupes manipulated by priests, and religion is useless since its myths don't correspond to objective truth).

Heck, there are even evolutionary studies on the benefit of religion to human development...