Thats just business though, why would you expect a home building company to pay £20k more for bore holing and a GSHP when they can install air source? All it means is your energy bills are several hundred more a year probably. You can extend that to anything in a house for that matter, why don't they build all houses with better insulation than regs, the best triple glazing, UFH everywhere, solar heating, PV, with everything required to make it a passive home? Theres no point as they don't reap benefits from energy savings and are then stuck with a house they have to sell for an inflated price to keep the same margins compared to if they didn’t. You will never get ground source over air source in new builds when the burden of cost is on a home building company.