3 reasons:
1. Materials: no brainer here, most are made out of plastic. This material degrades over time and any large force applied alters its strength.
2. New designs: crash test safety is ever evolving due to new discoveries, in both real life crashes and in the crash lab.
3. Changing Standards/Laws: Crash test safety is ever evolving. So much is discovered every year, its like a mad scientist race and the benefits from the 50's are hard to comprehend. Colonel Stapp did some crazy things to his body after realizing that his pilots were getting more injuries in car crashes than in military aircraft.
Cool video, and this was car technology 12yrs ago: