It is actually an insurance kind of thing as far as I understand. I've had plenty of cases where the bus couldn't continue due to being in a traffic jam, and people who would have happily get off the bus couldn't either.
But it just goes to show how idiotic this culture of strict adherence to rules can be. I've been more than once been forced to sit on a bus for 10+ minutes, with the bus literally stopped by the sidewalk, and the driver still wouldn't budge when I said "I can just walk from here".
It's feels worse than talking to a robot. At least I know robots have been programmed.
Whenever something convenient in germany is not possible, you can bet it is a insurance thing.