Telcos and cablecos that own the lines in the ground are not an ordinary company; they often have a monopoly granted by the city that prevents others from running a parallel physical deployment, which would be wasteful anyway. Further, the last mile physical deployments are often subsidized by taxes.

As you say in your second paragraph, removing the exclusive rights held by telcos and cablecos would be one option, but since running parallel cables is physically wasteful, I'd prefer common ownership or regulation of the last mile to save companies and customers the expense of running a parallel cable, while ensuring competition beyond the last mile.