Counterpoint: why am I able to buy a Ram 2500 and drive it without a CDL? I rented one recently to help a friend move some furniture and it's an unbelievably big vehicle.

The example of the standard you suggest could equally apply to large motor vehicles: you need special training above and beyond the status quo for vehicles that meet a certain standard. To your own point, motor vehicle laws were largely written based on outdated assumptions about the size of trucks and SUVs.