physical - Why don't most elevator panels let you unselect?

We've all seen it in the movies where a kid will push all the buttons in an elevator. We've all likely done it - accidentally hit the wrong floor.

There's no unselect after you select a floor in an elevator.

Why is that? It seems that it would be a useful feature but is there some sort of logical or UX reason why you would not implement an unselect feature?