My parent was not arguing they should be funding SENS to do good. My parent's premise was, why are these people not acting selfishly? Further, that funding SENS would be the most rational selfish choice.

Funding SENS for the sake of your children is not selfish, so it's irrelevant to the question of why are billionaires not investing in SENS to extend their own lives, as the rational selfish choice

If you care about your children, then hugely benefitting them is both selfish (changes the world in a way you personally prefer) and rational. Whatever the OP meant I think it's important.