Because they're actively discouraged from it. Kids try to play baseball or basket ball in the street they get yelled at. Kids wandering around minding their own business, they get stopped by cops and given the third degree. I live right across the street from a small park with a baseball diamond and basket ball court, there's a community garden on the corner, and down the street is a public swimming pool. They're absolutely swarming with kids as the first hint of nice weather! But heaven fore fend that they try to play a block over, someone is going to complain to them, or their parents, or call CPS to report abandoned/neglected children.