This comes down to multiple factors.
Firstly, humans are naturally greedy and competitive, you want to do better than the guy next to you. It's instinct.
Another factor is the fact that people who work hard want their hard earned money. Which means, for those who work harder and longer, they want more money, in turn making them wealthier.
A lot of people don't enjoy working, for whatever reason, so why should they get paid as someone who works a lot harder?
You'll find that those who are wealthy/rich have worked hard to get to where they are.
This is an extremely brief answer to your question, but I hope you understand it and learn something..Great question by the way!