Private and public philanthropy both contribute to science in the same way. If you're asking why private philanthropists can't replace federal funding, it's the same as the reason why we can't just make billionaires pay all the tax: the NSF budget would bankrupt Bill Gates in 13 years.

More realistically, what would happen would be that rather than sacrificing themselves for the greater good in some kind of voluntarily socialist outpouring of wealth, they'd ask us to look to China for our scientific future.