There are a lot of municipalities doing exactly what you say and making the money back fast. But a lot of states have blocked doing this, and the incumbents fig

There are a lot of municipalities doing exactly what you say and making the money back fast. But a lot of states have blocked doing this, and the incumbents fight like crazy to turn people against the idea. My favorite way of implementing it is open access. The city installs and owns the infrastructure and anyone can come be an ISP on it. You get great competition. It's basically the same as how it works with the community owning the roads and you pick who you want to service your house (FedEx, UPS, yourself, DoorDash, the local roofing guy, etc.) The other thing that would be worthwhile is local loop unbundling, which is basically open access but with existing providers having to open their infrastructure. I highly recommend muninetworks.org and their podcast if you want to learn more about municipal broadband.