I've wondered the same with basically any amp, home stereo, PA amp. They all start to distort, some with very unpleasant and even destroying results. (Have you ever blown a diskant horn of a lil' PA system?)
So why the engineers even allow us to turn the volume knob over half way? Just place a block to that point. I wanna be controlled...
And to take it a bit further. What if amp makers in the fifties would have given a thought to this and such a hardware limiter would have been a norm in guitar amps as well? Grady Martin wouldn't have got the sound like in Train Kept a rollin', Link Wray wouldn't pierce his speakers to emulate that and no one wouldn't have given a though to make a fuzz box.
But the question of why it goes to twelve. That doesn't surprise me. But how anyone dared to make an amp that only goes to ten after that?