Libertarianism is a fundamentally incoherent ideology. It only makes sense if you hold certain assumptions that just aren’t true. The core one of these is the assumption that perfectly free markets will naturally be competitive. The libertarian idea is that monopolies and oligopolies and that sort of thing can only result from government intervention, and that if there was no government every market would be competitive and efficient. This used to be a pretty plausible thing to believe back when it was first thought up in the 1800s. But now we know that’s just not how things work. Government can certainly intervene to create monopolies or hurt competitiveness, but they’re not required. It’s perfectly possible for businesses to use economies of scale and shady deals with other businesses to destroy competition in whatever market they’re in without any help from the government. And meanwhile, the government is the only entity with the power to break up monopolies, and prevent them from forming in the first place.
Competitive markets are an incredibly powerful tool for promoting innovation and efficiency. However, they’re also an incredibly delicate tool that needs careful maintenance. If left to their own devices they break and turn into a series of rent seeking monopolies and oligopolies. There’s a real debate to be had about the merits of markets versus central planning. What’s not really debatable is that if you’re using markets you need to keep them working properly or you have the worst of both worlds: one big organization without the pressure of competition running everything, but controlled by rent-seeking capitalists rather than planners who are at least trying to produce good outcomes.