And often metal farming implements dragging the ground, and a vehicle that sometimes makes your head the highest thing from the ground. Maybe not the best high level evidence that tires on a car don't help.
It's the metal cage of the car that provides lighting a more conductive path around instead of through you.
That's different than laying in a ditch or sitting in a car. At least in the car, the body is higher off the ground, and a lower resistance path than you. Perhaps a convertible, open top, bare metal floor with a flat tire might be comparable.