I spend lots of time walking through old, mature neighbourhoods with mature trees. Usually the sidewalks are empty, because stuff is too spread out to be walkable, and there just aren't enough people for sidewalks to be full. Yes, mostly in the US, but I've also observed this outside the US. Leafy+dense enough to be vibrant areas are really nice, but the exception. The thing that really makes new neighborhoods awful for walking isn't lack of shade, it's everything else about the new neighborhood, typically built in an extremely car-centric manner.