Because the government then by default has a monopoly on everything. The worst of the worst. It doesn't matter who the people with the monopoly are, it matters

Because the government then by default has a monopoly on everything. The worst of the worst. It doesn't matter who the people with the monopoly are, it matters that there is a monopoly. It's like saying that a just king is better than separation of powers. Maybe, but in which way do you ensure the king stays just? How is it that the means of production being privately owned for profit has the consequence of legislation failing to prevent monopolies? The government managed to break up the monopoly that Standard Oil had become back in 1911 just fine with capitalism existing, did it not?