Could be a number of things. Parking lots that are large enough would be for commercial buildings (i.e., office building or shopping centers). The owner of the building + parking lot might not want to be in the energy production business. And working out a legal agreement with the local power company might be too complicated. For example, a car hits a support for the solar panels. Or a solar panel falls and totals are car. If a panel on a farm falls and kills a cow the downside is relatively minor.
The technology is moving fast. The law and lawyers typically don't move as fast.
Urban parking lots are ephemeral. They are a way to pay taxes and make a little income before a large commercial business is built there. Perhaps the costs of constructing a solar park and the costs of removing it don't seem practical in the face of an uncertain future.
Physical footprint changes like this are sloooooow. For example, “according to the latest data from the 2021 American Community Survey (ACS), the median age of owner-occupied homes was 40 years.” Add that to every other problem you mentioned, and several more in comments, and even in places like Arizona where a shaded parking spot is valuable than typical you just don’t see a ton of installations.
Cost. The price increase is 50% to a 100% more then conventional. [0]
Some countries, like France, have passed legislation to make sure large parking lots have at least 50% of their lot covered in solar canopies. [1]
Other studies are trying to convince stores and owners of such places, that the investment is worth it and can possibly even take them of the grid.
[0] https://digitalcommons.mtu.edu/cgi/viewcontent.cgi?article=1...
[1] https://www.forbes.com/sites/carltonreid/2022/11/09/solar-pa...