Cell service is also highly monopolistic. It is honestly very strange we have 4 relatively competitive network operators in the US - it isn't a stable system to say the least, though, and it will almost always degenerate into two (and it almost did - for a while T-mobile and Sprint were non-competitive with AT&T and Verizon, and it took their complacency in the 3G->4G transition era to give the two underdogs a chance to reestablish themselves).
Fundamentally, though, nobody in the US can open a new cell phone operator now. The spectrum is bought and owned, the owners know how valuable it is, and owning light is a much scarcer and more valuable resource than any plot of land that billionaires are buying up in the present property investment bubble.
It is even worse than the ISP monopoly - ISPs are local monopolies. We have a national cartel of cell operators that can't face real competition because of a state guaranteed monopoly on radio waves. As is the nature of all business with guaranteed profits, eventually AT&T and Verizon will consume T-Mobile and Sprint, possibly even under this administration, where there will be much less anti-monopoly scrutiny, and then we will have two network operators controlling all the usable cellular bandwidth with no intent to let anyone else compete.
Just because we are in an... alright situation right now, doesn't guarantee that environment will persist forever. Cellular network operators are absolutely crooked and near guaranteed in a long enough time span to devolve into a duopoly, while we already have collusion between them today to screw over consumers.