why is it NOT?

In past 20+ years, Corolla is considered 1 of the best cars ever made in term of quality. Corolla is the top selling sedan ever in USA.

How many times do you hear run-away Corolla? or how many times do you see break-down Corolla in highway, compared to other brands or models? Even with this brake / engine / throttle things, statistically Corolla is still 1 of most reliable car. (safety is totally unrelated issue here).

If brake-overwrite can stop the vehicle when something bad happens, you will have another layer of peace of mind.