I think he said multiple times that he found it very hard to spend his money (for good things) faster than he's still earning it. He was the poster boy for capitalism in the booming tech market when it happened (with all the shitty business practices you can think of), but after he left Microsoft, he's been doing good things and trying to throw as much money as possible on it. He's at a level where his wealth hoarding happens on its own, even if he's not greedy for money. He's not trying to hoard it, he's part of the group of billionaires who keep saying they need to be taxed more.