alt Hacker NewsI am guessing: There is an evolutionary "shadow". Genes for getting old and healthy are not selected for, because you get old after having children. Evolution optimizes for the survival of your children.
Might be that cancer hits after creating offspring.
In a social species such as ours, with such a prolonged childhood, having healthy parents and grandparents is likely to affect the survival of children so there will be some selection pressure on a long life there.