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 tha

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!).