> why not build panels anywhere?

Because there's better alternatives same places. Norway has legendary well-suited topography for hydro power (>90% of electricity), and it's reliably windy (>8% of electricity). It's also so far north that the sun doesn't shine very much for half the year, and it's notoriously cloudy.

So yes, it will probably never make much sense to build a lot of solar panels in Norway. Same for Greenland, Iceland (substitute geothermal), and probably some parts of Canada, Alaska and Southern Argentina.

But also, yes, there's almost nobody living in those places. They're not terribly relevant in the grand scheme of things. Probably significantly less than 50M people in total.