Why is private philanthropy not sensible in this case? Should all philanthropy be socialized and centralized and administered by the federal government?

No, it should not all be socialized and centralized. Think things are running smoothly in the US at the moment? I wish it were the case that Michael Dell would have to consider whether the deployment of that kind of capital is a layup for him or if it requires some major sacrifice on his part. And, yes, it's better that he does it than not, but I won't pat him on the back too hard given the math.