Capitalism is explicitly designed for people to benefit themselves at the expense of others. Capital begets more capital in a positive feedback loop that results in massively powerful billionaires.
If you elect representatives, those representatives are checked somewhat by the threat of being voted out. Capitalism has no such check. Sure, ostensibly people can choose not to buy a product, but unregulated capitalism selects for monopolies.
This is merely a projection of a square on the surface of a cone projected onto a plane.