If we assume for a moment that this is true, then why would this state of affairs be okay? Society cannot run on the philanthopy of billionaires.
Further, there's not very much evidence that this is actually true. In terms of largest individual contributions, yes, many billionaires make impressive donations and pay for valuable things, but that's the all-too-common fallacy of looking at absolute figures when it's relative figures that matter.
Ignoring that a lot of charity and philantropic work wouldn't even be _necessary_ in a world with less extreme wealth inequality, the idea that non-billionaires aren't proportionally charitable doesn't really hold much water.