What kills you in a wreck? Several things. You answered your own question but don't realize it.
First off hitting hard things. Steering wheels, dash, windshield. That causes damage. Then there is the secondary impact of organs hitting the inside of the body cavity that they were never designed to impact. The delicate organs are torn, crushed, shocked into working poorly, if at all. Things also tear from being stretched, like blood vessels. The brain impacting the inside of the skull doesn't need much explanation, This is often a double impact. Coup-Contra Coup impact. Easiest explanation, it bounces around off the front and back (or sides) of the closed skull. There are the rare penetrations of the passenger compartment that cause damage, mostly unsurvivable, but some certainly have.
Being ejected from the vehicle increases your chances of being deaded by 75% Road, trees, sign posts, and the auto itself all become hazards. You don't have a lot of mercy heading your way outside the vehicle.
Best chance of survival? Stay in the seat you started out in. Seat belts keep you there, air bags dampen the soft you from the hard world.
I can't say I never unbuckled a body, but can say I have removed seatbelts from people who were alive that had no excuse what so ever for still being with this world.