Why? Seatbelts greatly reduce deadly harm for people in the cars and outside of it.

There are many occurrences where someone died because the body of a person in a crash who wasn't wearing a seatbelt just crashed into them.

Seat belts and motorcycle helmets shouldn't be mandatory in countries that don't have public healthcare, but they should be mandatory in countries with public healthcare because everyone else is paying for your medical treatment and seatbelts and helmets reduce the cost of your medical treatment.

And what is your logic behind that?

Is your next statement something like "Restaurants shouldn't be required to follow food prep laws or other laws like refrigeration temperature requirements"?