It's one thing for a working person to spend whatever income they don't need to live at a baseline on others, it's another for someone to hoard so much money they couldn't necessarily physically spend it all if they tried, let alone spend it on things that would actually measurably increase their happiness.

You can argue regular people should donate more, and many actually do donate more than billionaires (as a % of assets/income) depending on which source you trust to give a good enough picture, (given a lot of donations are hard to track, both from large billionaire foundations and DAFs, to smaller donors with hard to classify spending) but there is a massive gap in how much a regular person can donate relative to a rich person, even just as a % of their income.

If you live paycheck to paycheck, but have, say, $20 left over at the end of the month in actual money to your name that hasn't already gone to groceries, rent, etc, (and we assume you have no other assets), your net worth is $20.

If a billionaire donates $999,000,000 to charity, that would be the equivalent of that person donating $19.98.

Unlike that person though, the billionaire would have a million dollars in net worth, enough money to buy a house, while the regular person would have $0.02.

Even if these conditions aren't perfect, and you assume maybe the person has some more net worth than $20, the point still stands. A billionaire can give up almost all of their net worth and still have enough money to comfortably live, or at least meet basic living standards for the average person. For most Americans, if they lose their job, have any surprise bill, or don't make as much money as they expected to, they will instantly become homeless the next month rent is due, even if they give up none of their existing assets and just stop adding more money on top.

This is why "billionairism" (not a real term ofc) is such a damaging condition. It not only causes you to become obsessed with hoarding wealth that you don't necessarily need, but it causes you to do so at the expense of others you could readily help without experiencing any material downside in your everyday life. There is no reason to hoard so much wealth.

Money is just a means to get or do things. If you are not spending that money, and you have more money than you'll ever need to spend, that excess dollar value past your realistic spending for the rest of your life is just a valueless number to you. It's a number that will never impact your life, but it can impact others. Hoarding it is stupid and immoral.