During the course of my career I have seen the tragic results of what happens when vehicle occupants aren't secured by seat belts and car seats. It's like motorcycle riders not wanting to wear a helmet. Female drivers say wearing a seat belt wrinkles their dress and people who don't wear helmets say it messes up their hair. Think about it and you'll agree that neither makes any sense.
When there's a crash, there's usually 3 collisions that take place.
1. The vehicle collides with something.
2. The body collides with something. (usually the steering wheel)
3. The internal organs, i.e., heart, lungs, liver, etc., collides with the rib cage.