> but why can't you run both the new and old code side by side and if the actions they take are materially different investigate further?

Because there are no closed facilities that you can use to actually perform any meaningful test. You could test "in-situ", but you would need an absolutely _huge_ testing area in order to accurately test and check all the different roadway configurations the vehicle is likely to encounter. You'll probably want more than one pass, some with pedestrians, some without, some in high light and some in low, etc..

It's worth noting that American's drive more than 260 billion miles each _month_. It's just an enormous problem.