I think you're missing the historical context of how the "life after death" idea served as an utility, in many religions.

We today have laws and moral separated from religion and institutions that both teaches it to the young citizen and uphold it. But that wasn't the case for vast majority of the history.

How would you convince a tribal person that can't perceive something beyond "good for me & my family/tribe is all justifications required" to act collaboratively beyond that view? Especially if that attitude is also causing suboptimal behavior around him.

Introduce the concept of "good behavior" but there's no guarantee he will follow. Even if you introduced law & punishment you really have no efficient way to enforce it, back in the days.

So you introduce the idea that "if you behave bad,(or your children does) you'll suffer beyond your death".

Just so happen this simple yet powerful idea don't really scale with a complex world