Comment by throwaway_ocr

In addition to all the sensible reasons others have pointed out, if you crash at a high enough speed without a seatbelt you become a projectile. If you are in the back seat when this happens, you are most certainly a danger to those in the front seats.

If the seatbelt saves your life from an accident in which you were at fault, it is easier to prosecute and extract compensation from the living than from the dead.