I'd say it's more that many humans are exploitive and have found ways to use capitalism to exploit people, just as happens under every system. It's not an necessary part of capitalism. In theory, everyone works and adds to the economy what they can and receives from it in kind. But the world is not perfect, and people love to exploit whatever they can.

I'm personally a communist in theory, but a realist in practice, and see that communism is just as much or more prone to exploitation in practice. It's basically the same idea of "everyone works, everyone thrives" but with a community centered thought process instead of a self centered one.