That's largely because the subject of human life-span is more of a solved subject than most people know.
Scientists have learned that 82% of your longevity is genetically determined. If you had nonogenarian parents, then odds are good that you will be nonogenarian too.
The remaining 18% is affected by things like diet, excercise, and substance abuse, which means, if you had nonogenarian parents, but you didn't take care of yourself, you'll only live to ge an octogenarian, while someone with parents who died in their 60's is still probably going to drop in his 60's, regardless how well he took care of himself.