And even if you do all that, you are placing an equipment that is fairly expensive into one of the worst environment of a car: next to the ground, lot's of shock and vibration, even higher thermal changes, water and other liquids in very close proximity. These were also the reasons why the Smart ended up with a classical design.

Wheels need to be somewhat expandable otherwise you increase your cost. And what happens if you have a flat, has the spare wheel got a motor as well?