Why are the plants trapped behind the walls?

On my way to an Asian Mega-Cities urban planning conference at Tokyo University, I was struck by the streetscape outside of the famous campus. To the left is the brick-clad campus, enclosed behind a wall and covered in a mature tree canopy. The sidewalk is wide and echoes the campus with a brick in-lay and small hedge on the street side. On either side of the road are heavily pruned ginkos, still without leaves in March. Across the street from the campus are the typical urban residential and commercial buildings completely bare of leaves or green plant life in winter.

It’s wonderful that the Tokyo University campus is so well planned with mature green spaces. But I wonder why some of that plant life cannot spread across the street and out into the neighborhood.