Why is it, that home owners are allowed to have their trees, bushes and vines extend over onto the sidewalks?

In many cases a person has to duck, or walk in the street to avoid all this over-growth of limbs and debris.

I find this quite annoying. Walking around my neighborhood in Nachez is the same as walking an obstacle course.

What do you suppose could be done about this?

N. Commwerce St.