FTTH is outrageously expensive.
No, it isn't. What's outrageously expensive is quasi-government monopolies that are really massive healthcare/retirement benefits companies.
Fiber itself is cheap, as are the electronics on either end of the cable. Trenching can be expensive, but it doesn't have to be. Trenching gets expensive when it involves legal battles and navigating permitting thickets controlled by local utilities and governments. The actual act of laying fiber in trenches isn't very costly, and doesn't take much time.
Go look at the FCC National Broadband Map, which is really great, btw. Filter the whole country by Fiber and look at the coverage. Note how N.D. and S.D have extremely high coverage. Also note how these are highly rural areas, with properties separated by great distances. Yet you can get FTTH there. Given the excuse that's been offered for decades; that the US is big and low population density makes broadband too expensive, this shouldn't be possible.
Yet there it is. Why? Because those states are conservative, and they don't turn every feature of life into another balkanized quasi-government mess, complete with public-sector unions, bought and paid for union politicians with crazy union contracts that turn every public institution and utility into another retirement benefits operation that also happens to provide electricty/water/internet/whatever.
That's why.