Capitalist competition leads to monopoly by its very nature. You can "compete" best by gaining monopoly power and that power is self-sustaining. That power also enables company owners to pay their workers less for more work and will also operate internationally to impose corporate will on entire nations, with those monopolies tying the entire political system to their own interests.

The silver lining is that monopolies lay bare the practicality of central planning, a common goal of socialists in power. The system competes, eliminates its own competition, and still manages to more or less function via its own bureaucracies. Nationalize under socialists and you just cut out the redundant bureaucracies and begin making work respond to social need rather than profits.