Horizontal Territory Allocation is a common practice with Oligopolies with physical products (which the telephone wires and routing equipment they build to run the internet very much is).

Basically two or three massive companies simply don’t enter eachothers turf by unspoken agreement and they all get to benefit by not actually competing with eachother. They they can take turns raising prices in their own turf and know their customers have to physically move to get their “competitors” prices. As long as they never actually talk to eachother about doing it it is technically not illegal.

As for how they got the turf in the first place this mostly was small governments at the town and county level signing short term exclusivity agreements with a telco to run the initial infrastructure back in the 80s-90s when this was common. And many of these municipalities actively work against new telcos moving into the area long after those original agreements ended. You can always rile up some nimbys to bitch about construction noise at a small town hall and halt projects like this for decades. This is exactly how my hometown spent 8 years blocking fios in an area that only had dsl.

You tell a 40-50 something homeowner a three inch patch of their grass will be ripped up for just a week and they’ll drag their balls bare over fields of broken glass to show up to town hall week after week for 8 years to avoid it even if their isp quadruples prices in the same time frame.