For most of history, life was so fraught for so many people, they probably had to believe they'd be rewarded when they died, just to make their hardship bearable.
I, uh, disagree rather violently with reducing my religion to a Pie in the Sky concept (not that I can't see where you're coming from, historically), but don't feel up to discussing it, so let's just leave it. Because yes, we are on opposite sides: From a Christian perspective death is unnatural, and the reason we fight so hard against it is because it is alien to us - we were made to live forever...