It really does seem like a no brainer. They should already be close to other infrastructure and they provide shade for parked cars which reduces the energy spent cooling the cars when people drive away. Even if you make very safe assumptions about the total area of parking lots in the US and their sun exposure, the amount of energy they could generate from solar panels on average rivals the current solar output.

Plenty of rooftop space for solar.

Where i am in the eu, car parks are where its at. Apart from on dirty great field which is being filled with panels for some reason.