Earlier quoted context omitted.
But you are not legally required to use the volume control regularly in order to drive safely. You are not even legally required to have any kind of volume control in your car. You are legally required to have and use the turn signals. You are legally required to have and use the windshield wipers (because you need to be able to see the road when it's raining). Same is true for the horn and hazard lights - those are…
You're looking at this from the wrong angle. We don't really want to mandate physical volume controls, because they're necessary. We want to ban touchscreen volume controls, because they're unsafe. If Tesla want to deal with this by having no volume control, that's fine! Good luck to them.
If you want physical controls that actually are significantly safer to use than touchscreens, you need to lay down some ground rules for them as well (reachability, size of buttons, tactile difference from surroundings,...). And that rules out a "blacklist" approach (banning some particular undesired solution) and instead requires a "whitelist" approach (requiring a solution within a defined set of guidelines that constitute what's considered an acceptable solution).