"Arizona is flat, its roads are straight, and new, and well marked, and the climate is dry and sunny. It's the perfect test-bed for getting machines to learn how to drive cars."
Except the rest of the world isn't flat, with straight new, well marked roads where it never rains or snows.
If self driving cars are developed in only these conditions, they'll have to go all the way back to the drawing board to teach them how to cope with curved roads with inclines and less than perfect road markings. Where rain, hail, sleet or snow is a part of evryday life. Heck there may even be salt on the roads or even potholes.
Or something totally random that fell off another vehicle or blew on to the road from elsewhere.