Cost is the only reason. It would be very expensive to build out solar shade for all the parking spaces, even with the money making benefits.

The reason is that most of these parking lots are in retail spaces. These spaces often use a "triple net" lease that makes the tenant responsible for power costs, not the land owner.

The land owner doesnt pay any power bills, so they don't care about long term utility savings, and the tenants cant justify a huge upfront cost that will save them money on power bills, but one they wont own and one they won't benefit financially from if they leave that particular retail space.

In order for this to happen consistently, it would need to be a legal requirement. Any law passed to do it would have immense amounts of monied opposition from rich land owners.

They're doing something like that in France - all new and existing car parks with space for more than 80 cars have to have solar panels https://www.theguardian.com/world/2022/nov/09/france-to-require-all-large-car-parks-to-be-covered-by-solar-panels

We have a bunch of that around my area too. The first was actually a high school. I think it was some sort of proof of concept thing. I bunch of colleges have that now so I assume there is some sort of grant.

There is a college on the coast that is a converted military base with massive amounts of unused/parking space. They added a huge ammonts of solar to power a significant portion of the campus.

And everything I just said was at least 15 years ago. It works pretty well.

Adding solar panels for parking lots that are needed, like the ones at the aiports, is good. But, we should get rid of as many huge parking lots in the cities as we can