@Intensely_Human is correct. ISPs sign contracts with your city or county (depending on state/province laws) for a designated area. They are the sole provider of one type of Internet there. So you have one cable company and one phone line Internet company. The exception to this is the wireless companies that you buy your cell phone line from. Some cities may allow a second choice in one location but it's not common outside the largest cities.

From the customer point of view, when you move in you are told what cable company serves your area. Then you have a choice of cable, phone line, satellite, or cell phone. Our government pretends that choice makes it not a monopoly.

Also, municipal run Internet is explicitly banned in many states. So if a town doesn't like any of the options or no private company will serve the town, they cannot setup their own.

There's other options, but they're all MUCH slower. If you want a different ISP with comparable or faster speeds, you need to move. In my case, internet is bundled with HOA fees. And there is no other fast option available at my address anyway.

Those are actually just for show. We've let like 3 companies buy up all of our grocery stores too.

We're finding out that anywhere our laws say the government can hold rich people accountable or rich people should do something it actually means they can just do whatever they want. Even the hard line laws like price collusion have gone unenforced for decades now. And now that there is an (a single) enforcement action, it's a civil suit that's not even threatening to cost them more than they made.

It's not a full monopoly. You can choose another ISP, but it's just that in practice you'd need to physically move to a new location to make that change of vendor.