But the accidents are not usually single car incidents.
So, you are then leaving the driver of the other car having been in a fatal accident, because you did not want to wear a seatbelt.
If you want to drive on a private road without a seatbelt, drunk, whatever, that is entirely your choice. If you are likely to interact with other road users, then your choices impact others. (Note: your choices on a private road impact your family etc, but, you do not risk leaving someone feeling guilty because they had a car crash which killed someone)