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.

Same reason you try to save somebody who wants to jump from a bridge? Cost is marginal and potential benefit is huge.

Additionally if it was optional people would forget to do it more often even if they don't consciously choose to risk their lives for no reason.

BTW they are not only endangering themselves - they also endanger their kids.