Jim writes:

It all may be related to relative damage done by an aircraft with an
unconscious pilot.

A Hummer weighs more than a Cessna and can veer off the road at any time, even
in highly congested urban areas. And yet this doesn't happen, even though
Hummer drivers don't need a third-class medical.

Along the same lines, I don't know why motor homes do not require any
special training while scooters and motorcycles do.

As I recall, no training was required for motorcycles where I lived, but the
road test was non-trivial.