This is why you wear seatbelts:
 Rollover Crash Accident High Speed Rollover |
The car above rolls 3 times in 2 seconds. Cars are about 2 meters wide so the outside of the car is rolling over at 20 mph. The person inside the car experienced 10 g's of force pushing them up against the door and its broken window.
If a car were rolling over at 40 mph then the person inside would experience 40 g's.
There is no way to resist that kind of force.
Being thrown from the car is bad but even worse is been partially thrown from the car and the vehicle continuing to rollover while your head sticks out the window. The result is a broken neck and if you survive then you spend the rest of your life as a quadriplegic.