The thing you got to understand is that the energy of the crash has to go somewhere. The same energy will apply to both cars, the modern car will absorb a lot of it by deforming, the old car won’t absorb any in that way because it’s a hard piece of metal. And you have to wonder, what is more important to you, the car chassis or the people inside? You might as well ask “why do we put packing peanuts if nails are a lot tougher” or “why do we ship eggs in weird cardboard boxes if a metal square would be more resilient”