What if someone jumped to the line, or fell in the line, would the automated system be able to see it and break?
What if the doors closed with someone coat on it and that person was being dragged by the metro? Would the automated system be able to detect that?
My point is that: 1 - Sensors and processing are still not developed enough to account for many situations. 2 - Even when they are developed enough, it's still up to the programmers to find an algorithm that accounts for all possible situations, and in there, there will be always the human factor that can make errors.