Many municipalities expressly made contracts with cable TV networks to wire their towns / neighborhoods in exchange for a monopoly in cable access. Else the cable company did not have enough incentive (or pretended to) to lay the cable at all.

So the cable company owned the only copper capable of broadband internet, and also the underground channels to lay fiber in. They naturally upgraded their hub spots to allow Internet traffic, and laying the last mile all over again is prohibitively expensive, or even administratively prohibited, in a lot of places.

There is, of course, phone infrastructure! But Bell was a textbook monopoly, and pieces if it retained the infrastructure after its split. So in some places they can profitably reuse the copper they have to also provide Internet service, and sometimes you can even get fiber from them in newer houses which were wired with fiber backbone for the phone service anyway. In some areas the fixed-line copper is not good enough, and running fiber is too expensive because of low housing density.

In large proper cities, of course, the situation is much better, because infrastructure cost per reached customer is much lower.