It's your local government/state government negotiating for you on your behalf. Its a monopoly that is allowed to happen because of the nature of how cable works. Its more logical to have 1 publicly regulated monopoly and fewer utility lines than having a bazillion miles of cable being piped around our neighborhoods by 10 different competing providers.
It all depends on the area you live in. Comcast is one of the oldest cable companies around if not the first. Depending on the State or area you live in, you either only have Comcast or you have more options. Point being; if you live in the woods and there is Comcast, most likely you have no other option but Comcast