They’ve got to rely upon structural rigidity as there is no space for crumple zones front or rear to absorb energy from an impact. So, they “bounce.”
You’re right, they “bounce”, but long before that bounce has the opportunity to cause injury the occupants have likely already been hurt or killed by the rigid structure transferring all of the force (minus whatever little the airbag can contribute) to their internal organs. There’s no engineering magic here. To provide the occupants the greatest chance of survival, the vehicle must to the greatest extent possible slow the rate of deceleration to a survivable level. No crumple zone means that much of that safety margin is removed and your body comes to a much more sudden stop as a result. The only way to avoid that is with space between you and the object hit, which requires a larger vehicle.