I know it doesn't require much land, but there isn't any land available in population dense areas. Roof installations would be required for row houses and situations like that.

This doesn't look like an apartment building.

And the apartments near me have them just behind the complex, usually under the overhang from the roof. If you have a balcony or whatever, it'll be just below/next to that. Most apartments are only 2-3 floors anyway, and there's plenty of horizontal space to fit one per unit, or ideally you just include HVAC into the rent so you don't need as many units. The apartment I lived in had trees next to it, so they stuck them between the trees and the complex.