The rest of the giant vehicles on the road

Dumb weight for one. Structural design for another. Everything in between the teo axles has to be very strong since it keeps forces generated by the road from ripping the car apart.

On top of that, outside the axles, you're in the automobile's crumple zone - ie it's deliberately designed to fold in itself to absorb the blow of impact.

I don't like putting my mother in law on my SUV's third row - I'm loath to put my kid