Fiber is the most economical solution, it's compact, cheap, not susceptible to electromagnetic interference from thunderstorms, not interesting for metal thieves, etc.

Most importantly, it can be heavily over-provisioned for peanuts, so your cable is future-proof, and you will never have dig the same trenches again.

Copper only makes sense if you already have it.

Then why isn't it everywhere, it's been practical for over 40 years now.

It is everywhere in new development. I remember Google buying tons of "dark fiber" capacity from telcos like 15 years ago; that fiber was likely laid for future needs 20-25 years ago. New apartment buildings in NYC just get fiber, with everything, including traditional "cable TV" with BNC connectors, powered by it.

But telcos have colossal copper networks, and they want to milk the last dollars from it before it has to be replaced, with digging and all. Hence price segmenting, with slower "copper" plans and premium "fiber" plans, obviously no matter if the building has fiber already.

Also, passive fiber interconnects have much higher losses than copper with RJ45s. This means you want to have no more than 2-3 connectors between pieces of active equipment, including from ISP to a building. This requires more careful planning, and this is why wiring past the apartment (or even office floor or a single-family house) level is usually copper Ethernet.

I think our phone lines (the only buried cable here that can do data) are probably >40 years old. They're still selling DSL over it.

Coaxial "cable TV" cables, also sometimes buried, can carry data all right, at pretty high speeds, given right electronics.

I'm aware of that, but here there's no coaxial cable TV lines either. The only lines in our area that can provide data service are the copper phone lines.