Opportunity cost is only an argument when doing one thing prevents you from doing the other. What's the limited resource here that would prevent a cable company from hiring more technicians and laying more cable?

Put another way, how does banning improvements in New York suddenly make improvements in Montana more profitable?

Sure but what are you going to do with those technicians after they've installed the hardware in a sparse region? It takes a lot of man-hours to do the install, especially for long distances that are rural homes, but not so many man-hours to maintain

Well, these rural areas keep electing "free market" republicans, so...

Personally, your argument seems dubious but I'll answer you in earnest.

If your argument were true this map would look much different: https://www.fcc.gov/health/maps

Local politicians all work just about the same way when it comes to approving an internet provider. Attracting them generally has to do with business incentives, which means if the State and/or Federal government do nothing then you'll get what you have today.

Plus you have the ability to serve more people, at a lower cost.

I can get 100 urban users up for 1/4 of the price to get 20 users in a rural sub-division. This isn't just capitalism, it's basic utilitarianism.