As a billionaire, your only scarce resource is time. So surely I would make investments in immortality - surely all the tech billionaires, especially the old ones, would be making huge crash investments into mind uploading technology such as plastination, because it's their only hope and they have a realistic prospect of moving the needle with all their funds... so there are perhaps two explanations:

1) maybe they really DO do this but keep it secret because they think it seems vain (they would rather discuss their charity work)

2) maybe they have bought into the convenient lie (like almost all people do) that aging and death is natural and desirable