If you try to dig up all the sidewalks and driveways on any given block block, most of the residents will raise holy hell with their local elected representatives and try to shut you down any way they can. Much of this regulation is in fact the will of the people, and I'm not sure it's wrong. It is a significant disruption to people's property, and it's silly to lay redundant network infrastructure. It's a bad solution to a stupid problem that only exists because of ideological intransigence.
A while back a competitor appeared and tried to run another wire, but it got so mired in permits, regulations, and lawsuits that it gave up. It wasn't the cost of laying the cable.
No, it's due to private property owners. The regulations are mostly dealing with what it takes to bypass then, but in libertarian utopia, those regulations would be gone and the cost would be higher, not lower, for a new cable layer.