But roofs already being used, as roofs. They are of limited area, you can't work on them so easily as open land, each roof will need its own negotiations and ag

But roofs already being used, as roofs. They are of limited area, you can't work on them so easily as open land, each roof will need its own negotiations and agreements, each roof will be different, each building will need to be connected separately to the grid, harder and more expensive to maintain, harder and more expensive to inspect, on and on; it's massively inefficient compared to just doing it on open land.