Also if Germany sends excess wind/solar electricity to Norway, and the hydro lake is emptied slower because of that, then the hydro acts exactly the same as battery storage — no pumping required. However some lakes have a minimum rate of outflow: when rivers need a minimum flow for sports, ecological, or downstream dam capacity reasons. Also during dry periods when lake capacity drops, the ability to be used as a battery also drops. https://news.ycombinator.com/item?id=28898426
(You can say that some of them, like fission, rely on heavy metals produced sort-of in a star. But still that wasn't from our sun.)
Because central government has a history of riding roughshod over the wishes of the local people. Not everyone is keen to see the place they call home or a place they enjoy holidaying in drowned under hundreds of metres of water.
Pretty much all the easily (both physically and politically) developed hydropower in Norway has already been at least partially developed.
Nonetheless there is a lot of potential for new and improved hydro stations and the work is ongoing.
Here are some figures about further expansion: https://www.nve.no/energi/energisystem/vannkraft/potensiell-...
Norway has a ton of natural places for pumped hydro installations. In most of the world, installing dams involves displacing thousands or even millions of people and doing vast environmental damage. But due to Norway’s unique geography, this is typically not the case there. Norway has an absurd number of natural, deep, steep-walled valleys and fjords.