It allows one to disavow any sense of social reciprocity after becoming obscenely rich.
I was curious, so I looked through his Wikipedia page -- it says he donated $1m to the Hebrew Immigrant Aid Society in 2009 (which helped his family move to USA when he was a child). Even the NYT article notes that "The gift is small, given Mr. Brin’s estimated $16 billion in personal wealth" :D
(this is like you making $1m annually and donating $62.50)
Brin didn't go to every high school: he went to the one he did.
And maybe he had a terrible experience and thought it contributed nothing to his success... but that's kind of a dick perspective at a certain level of wealth, especially if a school has needs (and they always do).
In anything less than a fully-equalizing society, philanthropy still has a place.
(Said as someone who thinks higher wealth brackets, including my own, should be taxed more heavily)
The parent commenter put it well, philanthropy is just the rich convincing [America] that things are fine.
That's the thing, you can only have that kind of number for so many years before you start really not wanting it to get down.
And chances are they have been buying quite a bit of lifestyle by borrowing against that number. Because selling would strip them of that voting control you pointed out. Then they can't really afford the number to go down, because the borrowing is effectively a cascade, so in reality they aren't anywhere close to free in their decisions.
(but I'd imagine that they are quite capable of deluding themselves into believing that the decisions they have to take to keep the number up are what they actually want)
They can indeed do with their toys whatever they want. They just don't want to put up with the bother of other investors trying to get rid of them, or the orange guy not sending them a Christmas card, or having a little less than infinite money.
Even if that control is only nominal, of it comes at the price of anticipating every wish institutional investors might have and obediently following them to the (unwritten) letter.
Why shouldn't this be classified as a mental illness? Imagine a monkey hoarding more food than they could possibly eat, to the point that it lies next to them rotting away, while members of their tribe are dying from starvation. We'd immediately say that there is something wrong with that money, but why do we feel it is normal that some humans hoard an insane amount of money?
Having a billionaire who believes they aren't rich enough and need to make more money is like an anorexia patient believing they aren't skinny enough and need to lose more weight.