The usual narrative is that capitalism and perfect competition are synonyms. No one is a monopoly. Firms compete and profits are competed away. But that's a curious narrative. A better one frames capitalism and perfect competition as opposites; capitalism is about the accumulation of capital, whereas the world of perfect competition is one in which you can't make any money. Why people tend to view capitalism and perfect competition as interchangeable is thus an interesting question that's worth exploring from several different angles.
Lots of great points in here about competition, whether in business or academics, monopolies, and markets.