On a parking lot, not 100% is used by cars. Lots of space for driving/walking through and around.

The example above show the paring lot partially full with a lot of space for solar panel roads: https://goo.gl/maps/ZR5kNMZYGbE2

Also, in France, we're limited in farming space. Sure, it's going to go away eventually, while we outsource farming elsewhere, but replacing fields with solar panels is not as obvious of a tradeoff as it could be elsewhere in the world.