Because it takes 3-5x as much energy to heat rock than water, and at least twice as much as many other common materials. Sorry to say, even air is more efficient depending on the scale of time you're trying to solve for, though its ability to retain that heat is directly limited by the properties of the insulated box -- like a house, for example.

https://en.wikipedia.org/wiki/Heat_capacity

https://theengineeringmindset.com/specific-heat-capacity-of-...

https://en.wikipedia.org/wiki/Thermal_energy_storage#Heat_st...

You're going to be tempted to reference the last link as evidence in your favor, but it's very much the opposite. It's saying the advantage concrete has is its ability to be heated to higher temperatures than water. Except it doesn't get to break the laws of physics and still requires 3-5x as much energy input, which is why it's really only practical for large scale operations that can safely heat the concrete to extreme temperatures, using massive amounts of electricity that would otherwise be wasted due to low grid demand. They are still losing at least 75% efficiency in that process, but it's slightly better than losing 100%, as long as you pretend there aren't any environmental impacts of producing all that extra concrete.

You'll notice the first installation referenced in this section actually uses 1,000 cubic feet of additional reinforced concrete and an entire home's worth of additional electricity to supply a single home with 50% of its heating and hot water. That's a second foundation's worth of concrete, for perspective.

And moreover, as we've already established, this concept isn't new at all. If it were legitimately more efficient and more practical than alternatives, every home would already be using its foundation as heat storage. But they don't, because it's not.