House builders are told (unless they're literally building just one or two homes) to call BT at the start. For any large site, or where it's no trouble anyway, BT will run fibre to the new builds for IIRC free, the builder needs to make sure there are the right ducts etcetera in the ground, then a van shows up and blows fibre.
But guess how many people in house building remember to do that?
So instead they have the houses mostly finished and the sparky asks OK where do these phone points connect to? So that's when BT is called. And then BT will shrug and say you can pay our normal fee, you get copper cables, we can't promise how good the Internet service will be.
The part that's sad is people will stand there, talking to a sales person about their £350k house, checking things on their phone, and not think to ask - wait, this does have high speed internet - right? You aren't trying to sell me a brand new house that lacks Internet access are you?