Seatbelts protect you. Always wear one when you're traveling in a car, even if only for a short distance. You may sometimes hear some anecdote about someone surviving a car accident who was not wearing a seatbelt, and some authority saying that the person would have died if the seatbelt was worn. Well, if that's ever really true, it is extremely rare compared to lives and functionality preserved by seatbelts.
Why don't they make cars that only start if the driver's seatbelt is buckled? If fact, cars now have sensors that deactivate the passenger airbag when there is no passenger. That same sensor could prevent a car from starting if a passenger isn't buckled in, too. Sure, someone could still buckle the seatbelt and sit on top of it, but that could be made uncomfortable, and I bet that the measure would still be a helpful nudge.