Why don't we cover all rooftops in solar panels?

I was walking around the city one day and I noticed how much empty space there is on top of buildings. Why not cover them with solar panels? Would it even be feasible?

Most buildings in my city are soviet-style 10-story apartment buildings with a flat, mostly unused roof. The ground footprint of these buildings is a rectangle.

The average efficiency of solar panels falls between 17%-19%.

This is a map of solar radiation in Europe.

solar radiation map

Here, we get around 1,400kWh/\(m^2\)/\(year\), so ~3.84kWh/\(m^2\)/\(day\). A 17.5% panel efficiency ratio means that we can extract a maximum of 0.672kWh/\(m^2\)/\(day\). And this is before conversion and storage losses.

On average, I use 7-8kW / day during the winter and up to 15kW / day during the summer. Adjusted for surface, that's anywhere from 0.2-0.4kW/\(m^2\)/\(day\), so the solar panels would barely provide enough electricity for an entire floor.