"trivially easy to build and there are hundreds of thousands of potential sites in the US alone."
Yeah, but plenty of people don't live in the US and their geography is much less suitable for pumped storage than the American (or Norwegian) one.
For example, Finland is very flat, so are the Baltic states and most of Poland. So is Bangladesh, which has three times as many people as all the above together, and a nascent industry that needs energy.
In Czechia, we have rolling hills that are suitable for some pumped storage, but we built up almost all the good locations. In one case, an entire top of a mountain was cut off in order to make a reservoir:
https://cs.wikipedia.org/wiki/Dlouh%C3%A9_str%C3%A1n%C4%9B_(...