It’s because of Silicon Valley I think. We all got broadband with the first generation of technology in the 90’s. Since then, there has been so much government subsidies for rolling out internet infrastructure that no telco will invest in upgrades unless there is government money to pay for it. But most of those subsidies only apply to new deployments, not upgrades. We don’t qualify because we already have “broadband” (which here just means faster than dialup). So nobody is willing to pony up the cash for fiber-to-the-home deployments.
It's surprising that it would still need government subsidies to start with. I'm not really into the details of such thing that much, but around here (northern Europe) we're basically getting fiber everywhere, first in cities but later also in very rural and (by European standards) low-population areas, and there are no subsidies involved. Private companies compete and goes to communities (in rural areas) or neighborhoods (in city- or city-like areas) and try to drum up interest, and if there's enough interest they start digging. And in densely populated areas it's a no-brainer, they just do it. Due to competition (just more than one supplier and you have the competition) it's kind of "got to start this area before the competitor does.."
The fiber is also used for TV etc., with several providers on the same fiber, so I imagine that the fiber provider gets income from the competing TV providers as well. Could be part of the why. As for myself, I only need and only pay for actual internet.
I've had 1Gb/1Gb fiber for many years now, and lately fiber arrives in the most unexpected areas (long distances, few residents). It (the deployment, not the monthly) used to be more costly, but it's not anymore. And no public money involved. I know that it used to be, as an experiment, a couple of decades or more ago, but only in certain areas.
This problem is by no means limited to the US. Germany has that problem as well in rural areas and there government is also providing subsidies to expand broadband access.
Sometimes this can even be a problem in cities when the available infrastructure is at the limit. It's quite possible to move to an area, after having checked that good internet is available and suddenly the provider says no when signing up.