The vast majority of cars are being driven by humans who we expect to carry out an emergency stop if necessary. Replacing the human with a computer driver which is intentionally incapable of any kind of emergency stop and trying to justify this by relying on a human supervisor who's required to take their eyes off the road for much of the trip to stop the car is basically murder.

(Besides, relying solely on an unassisted human driver - even an attentive one - is dangerous enough that the industry wants to make automatic emergency braking systems mandatory on all new cars as soon as it's practical.)