I presume it is just a consequence of the enormous costs to build out a fiber network that would yield uncompetitive prices compared to the internet people are willing to accept from their coaxial cable internet providers.

A combination of people do not demand it enough to pay sufficiently to get neighborhoods wired with fiber, and coaxial cable companies having a disincentive to allocate more of their wire capacity to internet bandwidth since they want to avoid becoming a dumb pipe.

The only way out is taxpayer subsidized build out of fiber operated like a utility, such as the system in Chattanooga Tennessee.