It's central vs decentralisation.

The electricy is consumed in the houses and not on the empty land.

Parking lots become a win-win with electric cars. They also keep the cars cleen and sun protected.

And it increases installation costs by a pretty huge amount. Installing solar panels in a field is MUCH cheaper than bolting them to a roof or building a structure to hold them above a car park.

Plus, most solar installs are grid connected so a significant portion of the electricity tends not to be consumed where it is produced. It’s not as if installing solar is an alternative to grid connections for most practical reasons.