Ground level solar in a big city doesn't make much sense, they'll be getting a lot of shade- which significantly reduces the power generated. They've made new panels that are better with partial shade, but it's still crazy.
High rises are expensive to build. The reason they are built in the first place is that land is even more expensive, and expensive land militates for parking garages rather than open parking lots.
Most parking lots in big cities are not surrounded by high-rises.