But it's easy to build & easy to read about, so "why isn't it everywhere already" still feels like an open question.

It's a good question. But still...

> Two economists are walking down the street. One of them says “Look, there’s a twenty-dollar bill on the sidewalk!” The other economist says “No there’s not. If there was, someone would have picked it up already.”

Combination of very cheap periodic power and suitable infrastructure to supply heat energy is more recent phenomena. Supply that is very cheap power and demand that is capability to use that energy later need to match.