There are a lot of places in cities, even cities with a fair amount of trees like Chicago, where there could easily be far more trees. It would just mean taking up more sidewalk space, cleaning up leaves in the fall, petals in the spring, etc. Businesses start complaining about it.
And sometimes it could be trees but they think other things are prettier.
Here’s a bit of the Magnificent Mile in Chicago. Could those flowers be replaced with trees? I don’t see why not.