Because then people think it's a good thing. Have a system to tax people appropriately, and you never need donations.
It is still clearly a good thing. Even by your own account - until the tax system is taxing "people appropriately", this is the way that wealthy donors give back. I think you and many people who share your opinion have a distaste for the current way taxes are levied, but that distaste should not manifest itself in shaming public donors for giving back.