The problem with small-scale private philanthropy by the wealthiest is that it achieves little
The problem with large-scale private philanthropy is that it distorts spending in favour of wealthy individuals' pet projects. Worthy projects ought to be supported by everyone, according to capacity.
But don't confuse taxing 6% of someone's worth with 14% of someone else's income. There needs to be tax reform for individuals to capture capital gains, and corporations to capture revenues and prevent profit stashing.