And all anyone has to do to prevent being harmed in a car accident is to drive safely. No need for seatbelts.
That would be just fine, if you were the only person on the road driving.
Let's say you're sitting at a stoplight, minding your own business, not wearing a seatbelt, then someone rams you from behind at 50 miles per hour. Or how about you're driving on the highway at 70 miles per hour, then your tire blows and your car spirals out of control - in the middle of traffic. Or maybe you're driving home at night, stopped at a stop sign, then a crazy drunk driver comes out of nowhere and hits you head on. Or perhaps you're a senior citizen who suddenly has a heart attack behind the wheel, and the car goes out of control, and you collide into opposing traffic (which did happen in my own town).
Case closed. Wear a seatbelt, children.
10 points to the person who realizes how this can still apply to the topic at hand.