No, it's not a "natural monopoly." The primary issue right now is getting the legal right to use this infrastructure, which is protected by local city and county governments. Most of these local governments simply choose to not allow other companies to move in.

https://www.wired.com/2013/07/we-need-to-stop-focusing-on-ju...

https://tech.slashdot.org/story/17/04/12/0345217/tennessee-c...