We have antitrust law for that. But antitrust law today is simply overwhelmed by the huge number of monopolies and oligopolies the government has created in almost every sector. And courts don't have the power to correct the problem by reducing the regulations that protected the monopolies in the first place, making their remedies temporary.
I agree with the first part of your statement but disagree quite strongly with the idea that the government created them. Monopolies and oligopolies are the natural product of unchecked capitalism. Without government activily enforcing antitrust and breaking up monopolies you naturally end up with companies having a stranglehold on their sector.
You’re right in that government is a monopoly in most cases.
Even if that were true, by definition not every company reaches monopoly status, so all others can fail too, unlike the government, so this argument against the GP's claim does not hold.