A few reasons: Trees need a lot of space and the space underneath a sidewalk isn’t enough for long term life. They can die after like 30 years? This is tree dependent and location dependent.
Tree roots can destroy sidewalks making it harder for people to go over them. (Think people in wheel chairs)
Liability in terms of damage (have you seen trees after a storm?)
I should have mentioned this but usually stuff like this is planted in front of people’s houses etc. I wouldn’t expect a pine tree planted in one of those. Same with a palm tree.
I’m from Pittsburgh and there’s a lot of greenery projects and ecological restoration currently going on. Outside of the city, it’s very heavily wooded. But it’s slow progress.
Those giant algae tanks miss the large point of trees and their physical benefits and do feel like a tech bro solution looking for a problem.