If gardens grew on the roofs

What a cool idea, but its implementation is only available somewhere in a parallel reality. Or is it still not? Its like with tomatoes on the windowsill, only on a larger scale.
Thinking about the theme of "fantasy", I saw old buildings and some vegetation on the roofs and thought: would it be great if absolutely all city roofs of houses and buildings were used for mini-gardens and kitchen gardens?! Just imagine on the roof of your house there is a greenhouse or vegetable garden from which you can harvest! Own greens, some radishes or tomatoes are purely for the residents of this house! Let there be a special person who followed all this, and then divided the harvest into tenants.
Oh, so many roofs go to waste!