I think the point is, we're extraordinarily lucky that he is choosing to use his money for good rather than for evil (for whatever definition of good and evil exists). The idea that the public should not be involved with determining what counts as "public good" is super risky.

The problem with relying on philanthropy to fund the public good is that we the people are counting on the philanthropist to have goals that align with our goals. Imagine a world where the top two richest guys, instead of deciding to use their wealth to fund vaccines and education, used it to fund warlords and dictators. We could have just as easily gotten that dice roll instead of the Warren Buffet and Bill Gates world.

This is ostensibly one of the reasons "we" choose to democratically organize a government to fund the public good: So that we're not so reliant on the whims of a few rich people. It's to avoid the nightmare scenario where the worlds richest person decides instead to use his money for evil. Not saying that government can't use their power for evil, just that, at least in theory, we can vote in new government. We can't vote for different World's Richest People.