Locking Focus while driving for teen phone

I would be able to lock the driving focus, so that my teen can not turn it off while driving. What's the point if they can turn it off? I know that under the "screen time" content and privacy restrictions you can prevent changes to the driving focus feature, but they can still turn it off while driving. Why don't they make it so that it turn on when they get in the car, and turns off when they turn the car off. That can't be a difficult feature to add. Maybe make the map app available, but that's it.