Seatbelts don't magically stop major accidents from happening. But they definitely keep minor accidents from turning into major accidents.
They don't keep you in control after you slam into something head on or flip the car.
They keep you in control after you clip a deer, or hit a rock, or someone rear-ends you, or you swerve hard to avoid something, and that can be the difference between a good story and killing yourself or others.
Do they? I've been rear-ended, wearing a belt. There was no possible way I could control the car under the g forces.
If you've got enough side forces to pull you out of your seat, you've lost control anyway.
BTW, race car drivers know to take their hands off the wheel just before impact, as the front wheels hit they can jerk the steering wheel hard enough to break your arms.
It's true that if you're going to do performance driving, tightly belting yourself in will enable you to feel the car better, and enable you to concentrate on driving rather than trying to stay seated. But hitting something is a whole 'nuther story. If you haven't been hit hard in a car (I have), you're not in control. Belt or not. You're just along for the ride.
They don't keep you in control after you slam into something head on or flip the car.
They keep you in control after you clip a deer, or hit a rock, or someone rear-ends you, or you swerve hard to avoid something, and that can be the difference between a good story and killing yourself or others.