Local ordinances specify minimum space requirements for trees, which may mean that they're not allowed to be put in certain places. Also, they can cause pedestrian safety issues, as well as Ada compliance problems in confined spaces. This is an easy way to get something green in a place where you would otherwise not see a tree because of a lot of beaurocratic bullshit.

Obviously, you can argue that all that needs to be changed. And you'd be right. And in many places it's moving that way. But then you also wouldn't get anything done for quite some time. This is an option where there might be no other viable options at the moment.