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?

One way councils can force the issue is to deny planning permission unless builders can show they've engaged a telecoms company (doesn't have to be BT, could be Virgin or Gigaclear or Cityfibre). After all, we don't accept houses built without electricity and water. Why should we accept houses with subpar connectivity?

I recently got a new fibre connection (not from Virgin or BT) in my newish flat and I was taking to the chap who installed it about exactly this. He told me that it's actually the builders who pay BT and Virgin to come and connect up their buildings. This is so they can then market them as being almost "plug-and-play" with these services as some buyers look for this. The other smaller providers can then come along and install/connect their stuff but at their expense, and as you say, often afterwards. He said that this is made harder since installing afterwards means dealing with managing agents (in the case of of a block of flats) and this can be painful, which I can 100% believe. Perhaps the situation varies depending on where you are in the country, this was London (where there are no £350k houses!).

One of my friends parents a couple of decades ago brought an architect designed house and the designers/contractors neglected to put in any ducting!

Took them ages to get POTS - You can see why Civil engineers make Architect jokes

My new house (well 2 years old now) came with fibre into the hall cupboard (and cat5 to every room), I'd assume that FTTP is default with newbuild

Most ISPs don't seem geared up for FTTP, so it's good for BT, as most people will just go with them.