Because its not a freely competitive market. They didn't acquire the infrastructure and access -- particularly the last mile access -- by freely negotiating with property owners, nor, even if they had, is there unlimited capacity for new competitors to do so. Incumbent major broadband providers all got the basic access essential to provide infrastructure as CATV and telephone providers, as locally or regionally regulated monopolies, and there is pretty much no way for direct competition with fixed broadband providers on equal footing.
Fixed broadband providers aren't an ideal market for the same reason "road providers" or "sewer service providers" aren't.
> If the Googles and Facebooks of the world don't want to abide by the terms set by the Comcasts of the world, they should build their own networks.
Google's been working very hard on that for many years, because they knew that their preferred regulatory framework wasn't something that they could count on.
OTOH, I don't see why the Comcasts and AT&Ts of the world now ought to be free to capture all the profit from business done over the internet any more than the AT&Ts of the telephone age were allowed to do so for all business done over the telephone.