"Though having plants is of course just as nice if not nicer."

A deer committing suicide from 10 stories that could be troublesome.

Painting roofs white paint is less problematic. Having numerous ecologies or parks on rooftops could be more fun than what is wanted.

We could live in stealthy cities- all roads and buildings covered with greenery. Perhaps it will become a fad 200 years from now.

But since Walmart wants to be seen as nicer, having a green roof could be better than solar panels.
Of course if a Walmart or say Starbucks has a green roof, the question become should the public have access to it- public access to a solar panel roof isn't normally something that immediately comes to mind. One could simply limit it to employees.

Schools could have playing fields on their roof.
With green roofs and large projects, one could have as a purpose the creation of city parks- though having trees makes it a bit more challenging. In city planning one could have a parking lot with park above it. And combine this with train service. So people could park their car to use a train, people could get to the park via train or car and one allow local residents a place to park their cars [and bikes].