Yeah trees are great wherever they'll fit, and in my region you pretty much have to work not to have land return to forest. But there are a bunch of regions where shade cloths or latticework are the traditional answer to shading streets for climate/biosphere reasons. Plus people get all freaked out about tree roots messing with building foundations and underground infrastructure so not all pedestrianized streets may be suitable depending on what's below them.

(Apologies if your second paragraph wasn't about trees)