If you put solar panels in parking lots, you're adding a lot of posts to the parking lot to hold them that weren't there before. People are going to crash into those posts in parking lots often.

For example, light posts constantly get bumped in parking lots.

To hold your solar panels you need really strong posts that can both hold them and get bumped into by vehicles. Especially in the USA where you have giant vehicles & tight parking spaces.

This all adds to the cost before you even get to electricity storage & transmission.