Most economical locations for hydro generation have been built out already. Pumped hydro doesn't require flow like generation does, so there are thousands of times more suitable locations. Those haven't been built out.
Maxion
At least in my country they have. They keep trying to build a facility but anything they come up with, the ROI just isn't there.
bryanlarsenOP
In other words, there are lots of places they could build, but it's just too expensive to build and/or the value of long term electricity storage is too low.