They do not do that. It's illegal under federal law. Pole and conduit owners are required to rent out access at non-discriminatory rates: https://www.law.cornell.edu/uscode/text/47/224.
The reason companies don't overbuild is because it's expensive and there isn't any return. FiOS came to my building in Baltimore. I was the only person on my floor to switch away from Comcast. Even these days, people choose their broadband provider based primarily on the TV package.
But they most certainly can impose enough bureaucracy and other hurdles to make renting factually impossible. Or simply both the incumbent provider and the county employ just a single FTE (or less!) to handle permits, and one can't do anything about it. All while following the letter of the law, because there's nothing in the law that says "county has X days to deal with the permit else it is being automatically granted".
Nothing in the US is "easily" litigated, especially not when you're up against a multimedia conglomerate and a portion of the government itself.
Those two words do not go together.
They definitely do. Our city has a franchise agreement with the incumbent cable provider; they're the only company allowed to provide cable television service. ~10 years ago a company called NuNet tried to come in and run fiber in a neighboring city, Hazleton, and both they and the city were sued by the incumbent for breaching the franchise.
I imagine there are some 30-year exclusive franchises granted in 1991 that are still in effect. I don't actually know of any.
Was FiOS much cheaper/better than Comcast (apart from the underlying tech)?
In the US AT&T and Verizon seem to have given up and want to just do cellular to make money and let their copper plant rot. They are gifting the entire market to Comcast et al.
The Telco can offer a few things like fiber connectivity between county buildings, tax payments, etc. By doing that, they secure the rights to the area. Given the federal system of the United States, this is a difficult thing to stop.
This makes true market-based competition within most areas very difficult to achieve.