Capitalism tends to reward competitive companies by making them larger. Large companies are fewer than small companies so choice among them is intrinsically reduced. It is more profitable for large companies to approach the traits of a monopoly so under capitalist forces alone, they do. Collusion among companies collectively produces the traits of a monopoly so under capitalism alone collisions will automatically arise. It is in the interest of monopolies to obstruct competition which may displace them, so they do.
What factors do you believe will compel large companies to act in the interest of the people if not something like government? It certainly isn't capitalism. Capitalists (incl. AnCaps) and free market believers have the assumption that a high degree of competition is the stable point for capitalistic systems. It is not. High competition represents an immature market and is a transitional period. As all markets mature, they arrive at ever increasingly stable monopolies and collusive collaborations.
Some force beyond capitalism alone is required to coerce quasi-monoplies to act in the interest of the people when the inevitable corruption of competition unlinks the interests of the people from the goals of the company.