Why? Studies show that people not wearing seat belts cause more damage to other people on the road, mostly because when an accident starts, they lose control easier. Plus, if passengers don't wear seat belts, they get killed by drivers. I would argue that the seat belt law saves more lives than the drivers license law (without evidence, it's just my feeling).
If the state has the authority to tell me I can't be driving a car on their roads without a license, they certainly have the authority to tell me I can't be driving a car on their roads without seat belts, or holding a cell phone, or eating and drinking, or listening to the radio, or having a conversation with someone in my car, or with my child sitting in the front seat, or in a car without an air bag, or whatever other stupid rule they think protects me from my own apparently stupid self.
If there were some other sufficiently onerous enforceable consequence to driving while stupid, then the license system wouldn’t be seen as needed. I’m not sure we who drive some kind of motor vehicle on public roads, at least those large enough to cause serious damage to other occupants of those roads, would want the necessarily draconian side of that sort of system. (It used to be in a lot of places that motorcycles didn’t need any license.)