Bluntly, they need the money to make more money.

Most people of any significant wealth have a majority of their money/assets tied up in investments and other things. They're worth that much basically on paper only. Their actual liquid monetary assets are generally very small in comparison to their "worth".

They're generally taking a relatively small cut off the top for their personal spending, and reinvesting whatever else they have "earned". Once you get past a certain point with wealth, as long as you're not stupid about it and ether invest it yourself or hire a firm to do it for you, more wealth is a normal outcome of having wealth.

There are calculations, as you may expect, to perform payouts "in perpetuity" (aka "forever") which can account for growth/inflation. Personally, if I suddenly became ultra rich, I would find a good balance between how I live now and how I want to live, estimate what I would need to earn to acquire that lifestyle, then do the calculations to find out exactly how much I need to hold onto to achieve that, then either donate or otherwise give away the rest. I'd keep a healthy buffer on how much I'm keeping, and likely use some of it to update/upgrade key items in my life, not lavishly, but something better than I currently have (renovations to my home, upgrade for my vehicle, computers, electronics, etc)... Once my needs, both immediate and in the future) are satisfied and I have that extra "buffer" taken care of, the rest is useless to me.

I'd likely start with large donations to causes I believe in, and gift large sums to friends with coaching on how to make that money work for them as I did for me. My friends and I could become financially independent with a large enough financial "win".

I don't need more than what I require to maintain a fairly modest lifestyle. I don't like, want, or desire any "glamour" or "fame". I mainly want to be left alone. Paying off my house and having a good amount of passive income from investments is sufficient for me (where passive income grows with inflation year over year). Beyond that, I have no use for wealth. I only want enough that I can make that money "work" for me on the way that I'm no longer required to work. If my investments and buffer result in significantly more than I need being generated, I would give that extra away.

But I'm more communally focused than anything. Helping my friends and neighbors is more important to me than the idea of "more for me".

To circle back to the point, wealth is the ultimate indicator of success in capitalism. People with excessive wealth are seen as some of the most important and influential people in a capitalist society. So to their mind, they're important because of the money that they have. The mere suggestion of giving most of it away tends to be taken as handouts to those who haven't earned it. Artificially inflating that person's statue in society while diminishing their own. They're at the top of the ladder of success in their mind, giving away their fortune puts them lower on the social ladder of success and therefore it's unacceptable. The wealth they have is the representation of their importance, and by diminishing it, even a little bit, they're "moving down" on the ladder of success, which is something that they never want to do.

I don't agree with that, but I understand it. I'd personally gladly sacrifice my own excesses to bring people up. I only want for enough to secure my ability to provide for me and my family. Anything beyond that isn't useful to me. I have no illusions of grandeur that because I have money I'm somehow better than anyone. I hold the opposite view, where earnings well above what is required to secure yourself financially, are unnecessary, and shouldn't be kept. We're all supposed to be equals in a democracy, but people who are aggressively capitalist see it differently.