All of the above answers make perfect sense. Just to expand upon Dom Tuk's answer:
Car manufacturers used to think that they could improve passenger safety by building their cars 'like tanks'. These days they deliberately design the common impact points (e.g. the front of the car) to form 'crumple zones' so that they absorb much of the impact of a collision. So, if it was possible to design an impact-proof plane, your chances of survival, in the event of an accident, would be lower (rather than higher) than with current designs. (Even if seat belts were changed to the full-body harnesses used by rally drivers, this wouldn't prevent neck injuries. Neither would it prevent your internal organs from continuing to hurtle forwards, at hundreds of miles per hour, when the rest of your body came to an abrupt halt).
Chris