In the specific case of ISPs I think it's always because you won't make enough money to justify it as a big company, yet the task is too big and complicated to do it as an individual nerd.
The worst part appears to be the physical wiring. If your government has implemented loop unbundling, you're already set (probably need to do some bureaucracy and pay some affordable-at-a-stretch fees to get access to it). Otherwise, or if the loops are just crap, you have to figure out how to physically get a cable to everywhere, a task that is fundamentally laborious and legally fraught, not nerdy at all (unless lawyers are nerds) so nobody wants to do it.
Wireless ISPs are about as popular because of this. Wireless service is always worse, but you only have to install plant (physical infrastructure) at the customer's house and one central location, not all the places leading up to the customer's house. This makes it a whole lot more amenable to individual-nerd or handful-of-nerds setup.
I encourage everyone to at least think about how they would do it.
>Wireless service is always worse
Look, wireless service is almost guaranteed to be worse, but that has more to do with dodgy operators. The technology is fantastic, and when engineered correctly largely undetectable.
That said, in my time, I can count on one hand the number of installations where I was allowed to engineer the service correctly. And I can count on all the hands in a small city the number of times I have been called to rescue something extremely stupid, like shooting a link across a construction site.
> ...you only have to install plant (physical infrastructure) at the customer's house and one central location, not all the places leading up to the customer's house.
In a rural environment, yeah, sure. Based on what I'm seeing in San Francisco, in an urban environment, you're going to be negotiating for roof space for many transceivers on many separate roofs. (I do absolutely agree that even that annoying tasks is certainly way less work than dealing with a local or state government that wants it to be impossible to run fiber through or along streets and sidewalks.)