It's because of design efficiency. The most distilled and finest design utilizes each part for many things. Like a unibody car, or a fuel tank in the wing in

It's because of design efficiency. The most distilled and finest design utilizes each part for many things. Like a unibody car, or a fuel tank in the wing in an aircraft. In this case the material doing the thermal storage is also part of the building. It's not like there is a building and a separate 100 tons of thermal storage stuff, just make the house and foundation out of that 100 tons of stuff and then you only have to buy 100 tons of stuff, not 200. It's the same economy of scale with the labor and digging. You can dig 1 hole and put all this geothermal stuff in it along with the foundation, or you can dig 2 holes and have twice the everything. If you just combine all the items into one big effort, look for design efficiencies where you can combine parts and systems, and these systems/buildings can basically run themselves with just what is provided by the environment. They don't change the environment inside the building, they just regulate it between the 2 extremes, taking the edge off the high and lows.