@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.