Why would people do that?

I think the root of most religious belief is that people want to be believe that death is not an end, so they create religious theories that their essential essence will go on in some form (heaven, reincarnation, as part of some universal soul, etc). There's no doubt that this belief is often taken advantage of by secular and religious leaders, but I think it is more like how con artists find their best targets in people looking for easy money. They don't create the desire, but they take advantage of it.

But that is why I'm a little surprised. In the absence of a regional religion, I think new ones will often spring up to meet that desire for eternal life.