If you force rich people to give away a large enough percentage of their money, perhaps you will remove any incentive for the non-philanthropic ones to keep earning more money. If they weren't inclined to give it away in the first place, they probably won't be inclined to make more, knowing they'll be required to give it away.
Because those people are already giving most of their money away to causes that benefit the world. The "other" rich people are still, on average, creating wealth though, via the companies they start and run. If you disincentivize them from doing those things, you eliminate that wealth, leaving everyone worse off on average.
