Show me a monopoly in the United States that isn't enforced by the government
Sure, I want to play the top of the line computer games. AAA titles. Because let's say I'm a gamer.
Now which operating system do I have to buy? Do I really have any choices?
Diamonds. Debeers had a global diamond monopoly that extended to the USA. Huh, this has apparently gotten better in the last few years.
Monsanto has the seed market. But oh, let me guess. Because the US government enforces IP laws, that means that the big bad evil guy is actually da gobermint.
Companies have no power to enforce a monopoly
...Other than market dominance. And all the things they can do to keep competitors out of their market. Remember "embrace, extend, extinguish"? Come on dude, your'e on slashdot, are you just going to pretend companies don't do things like this?
You can claim that there are some natural monopolies, but if these are actually natural monopolies, then why would it require a law to prevent anyone from competing with them?
Yeah, some natural monopolies, like the power grid and the cables connecting my house to the grid.
Your question is backwards. They don't have laws keeping out competition (pft, hell with all the regulatory capture, they probably do, but hey, I'm talking about in general here) they have laws regulating HOW that natural monopoly is used. Someone wants to buy a section of the power grid? Go for it, but the regulation applies to them just the same.
The "natural monopoly" of the power grid is that there's just the one. Alright? I don't have the choice to connect my house power to a number of competing grids.
Even if a capitalist managed to achieve a local monopoly on something, the only thing keeping their competitors away is if the barriers to entry are larger than the potential profit.
"Even"? "EVEN!?" That by itself is some pretty bad mojo. No competition means shitty service, no price competition, and the consumer can suffer the entire cost of the barrier to entry, every year. If anyone tries to enter the market, the monopoly holder drops the price down to unsustainable until the new guy is dead and gone.
And in case this concept is entirely foreign to you, companies strive to increase the barrier to entry for their market. You're absolutely right that one of the ways they do this is by getting the government to pass regulation that is difficult to meet. And when those regulations dance to the monopolies tune, that's regulatory capture, and it's a terrible thing. But don't pretend that the government is the root of all evil, or companies are all saints, or the market will cure all ills. It doesn't always work everywhere. Like essential utilities with natural monopolies.