I got interested in this tech a while back myself (thinking I should build one myself, how hard can it be?) and I made some simulations (because who wouldn’t). The thermal gradient in sand is actually quite steep, even with very high temps around the core where the resistors are, the temps fall off pretty quickly. Then when you are at a point where the surface temp is reasonable, you switch to a conventional thermal insulator. Putting it underground would cause all sorts of issues in eg. maintaining the resistors, insulation etc. so it’s probably just easier to do it like this.

No, but they exist.

Turbines are still kings in thermoelectric generation.