If we want a shift to walking, we need cities to plant around 100x the number of trees they have. Ever walk through an old, mature neighbourhood? Usually there are tons of people on the sidewalks, and a primary reason is that there are mature trees providing plenty of shade. Then try walking in a new neighbourhood with barely any shade. It is awful.
It's like homes used to be so much closer to the sidewalk, it was just a couple of steps to reach the sidewalk and get going, but now it's these giant football field widths separating homes from the sidewalk, and then massive 4 lane sized roads separating sidewalks on either side. I'm exaggerating of course but the point is still there, the scale is just so different planting trees won't solve it.
This difference in scale creates such a different atmosphere, where sidewalks are just for dog walkers and bored baby sitters, not for regular commute. It's like if you want to talk to your neighbour from the sidewalk you have to bring a megaphone.