Why do people need an incentive? Why can't people just raise their kids, maintain their communities and generally just live a simple life?

Why do they need to strive? They don't. They totally could be happy as you describe. But you know they won't be. They aren't wired to be happy with a little because they won't value it because they didn't work for it.

For a depression-era farmer even a little success was still survival and so it mattered. But for someone who doesn't do anything to produce their own goods and so never feels accomplished because of skill or effort, anything less than what the other guy has will be intolerable. They won't work for anything, so they won't accept hard work as a reason why the other guy deserves to get to keep what he produced, and they'll steal it, killing the goose that laid the golden eggs.