It's kind of his money that he controls. He can do a lot with it, but he can't buy a yacht with it... You don't get the favorable tax treatment without a ton of restrictions on how you can spend the money.
Think of it this way: Gates has a certain list of things he wants to spend money on. The government likes some of those things and forgoes taxes on them.
That doesn't change the fact that Gates is spending his money on exactly what he wants to. The restrictions are essentially irrelevant to him.
Bill Gates would never need to buy a yacht with his foundation money because he would buy a yacht before he made it foundation money.
It's like saying I can't buy a new TV with my retirement funds, but, I can certainly buy a new TV and contribute less to my retirement funds -- the end result is the exact same dollars went to the exact same end, I just organized them differently.
A version of this is that every kid who got scholarship money or loans knows how to bucket expenses to use your "limited use" dollars on qualifying expenses that would otherwise be paid by general funds, freeing up general funds for things. The net result is that the loans paid for nonqualified expenses, but they were organized appropriately.
I mean, you could say that groceries are part of your net worth, and immediately after purchase are roughly similar in value to the money you traded for them. Obviously people don't tend to bother counting their groceries as part of their net worth, mostly because they go bad or are consumed quickly and comprise a tiny percentage of most people's net worth.
But, in general, yes, your net worth is the value of everything you own (minus your debt). It's not at all crazy to say that, when you buy something very valuable or transfer money to a foundation which you control, your net worth is not changing much.
Yes, but that point comes when you eat the groceries, not when you buy them.