Xfinity has a massive market share and is often the only game in town or one of only two. The only alternatives are often smaller resellers of the exact same infrastructure under a different company name. Or, there's also satellite Internet, but it's an order of magnitude slower, has a low data cap, and costs at least as much as the others.
Sometimes, the situation is so bad that people start their own ISP rather than suffer the exorbitant prices and lackluster support:
