Proper protection in a car crash means two things: you must be safe from being crushed directly, and deceleration must be slow enough to keep you from internal organs injuries and injures to your neck. The first is taken care by firm safety cage of the car, and the controlled crumpling/deformation of the front of the car slows down the crash taking care of the second case. More time = less deceleration = less stress on your body.

Sturdy cage only is not enough—you would not want to be dropped from sixth floor in metal safe, would you? The safe itself may survive the impact "without a scratch", but that would not help you much…