You can either have regional monopolies or the ground constantly being torn up by companies wanting to lay their own cables, resulting in a clusterfuck for any other construction effort, and there's only so much space in the ground for cables anyways, not to mention that it's not practical for new players to enter the market given the costs of laying cables, putting them at a competitive disadvantage.

Sure, only 21 states codify telecom monopolies into law, but in the rest without those laws, how much more choice is there? Let's not count the states where the government paid for dark fiber which is leased to the corporations to provide service for customers, because that seems to be taking the correct route. If that were to happen in other states, it would necessarily require either (1) taking over the telecom infrastructure and leasing it back to them, or (2) expensive buyouts of the telecom's infrastructure. It seems simpler and cheaper to regulate a monopoly in this case.

EDIT: Correction: the laws in these 21 states don't codify monopolies per se, but they make it impossible for cities to build their own infrastructure on some absurd basis that the cities are anti-competitive with respect to corporations:

https://motherboard.vice.com/en_us/article/qkvn4x/the-21-law...

EDIT2: Ignore any argument I was trying to make, as I may or may not have contradicted myself based on my mental shortcomings with regard to concentration and lack of medication.