fruitful city planning

Lots of cities have grassy medians on roads with trees planted in them, or on the sides of streets, in parks, etc.

Since the basic cost for planting a tree is more-or-less constant (at a given size), and the cost of differing types of trees tends to be similar at any given size, why do cities not plant fruit trees along their byways?

Yes, fallen apples can make a mess on the road, or bonk a passing pedestrian, but they could also provide a lovely perfume to the city, and offer “free” snacks to those willing to take the few seconds to pick one on their way by.