The ISPs only have a monopoly because the government literally makes it illegal to start a competing ISP in many areas. Even local governments were forbidden from competing with Comcast et al. in some cities.
They can out lobby you. They can undercut your pricing. And they cut "selectively upgrade" areas that would be profitable to come into.
Google couldn't cut through them. Mainly because as soon as Google threatened to come somewhere *WHOA MAGIC! POOF!" and the ISPs suddenly had orders of magnitude more bandwidth for half the price. Funny that.
That may be how they gained their monopoly, but they keep their monopoly because overbuilding an additional network is very expensive, very tedious (need pole access), and very hard to sell (it's going to be hard to convince most people to join because the internet is better, people think it's a commodity). And, incumbents can drop prices to keep you out, while keeping them high elsewhere.
