I find the prevalence of faith makes more sense if you think of it like a living organism. It only exists because it's built to exist. If it didn't, it would die.

That's why faiths often have rules around birth control and sex out of wedlock. Kids often take the beliefs of their parents, so the religion has to keep 'traditional' families together to keep itself alive. It's also why they threaten eternal damnation if you drop the faith or don't try to force it on the people around you. A lot of this often isn't the conscious effort of the members, it just kinda slowly crops up, like evolutionary mutations. Key word there being 'often', as I'm sure members of these religions have also figured this out but have used it to their own advantage.