If this is true then how can we have had Collision Avoidance systems[1] in cars for years that are able to automatically apply the brakes to prevent this sort of accident? And If the answer is 'autopilot doesn't work that way' then why not use both systems, with the collision avoidance system taking precedence as it deems necessary?

1. https://en.wikipedia.org/wiki/Collision_avoidance_system