What happens when there's a fire along the highway (e.g California)?

What happens when there's debris and objects in the road?

What happens if a car needs to make an emergency stop?

Where do you place these new roads?

Do they require roads to be re-built or can they be added to existing roads?

What is the cost of replacing the entire highway and city infrastructure?

The AV companies decided that the simpler problem is to have AI that works on highways, then incrementally teach it how to handle more complex situations. This is the most additive solution, whereas your solution is not necessarily additive and does not necessarily preclude the need for AI.