But they'll spend less of it on the public good.

The government may be less efficient (also, citation needed there, as I don't buy that at face value), but even if it is it has access to WAY more funds.

Sure, we have some nice billionaires like Zuckerberg and Gates who give their money away, but if you required all the billionaires to give up some money, you'd have a much larger pool to work with. So efficiency isn't the only part of this argument. There is also volume.