Money is money whether it is invested or not. Just because your money is being invested instead of being in a locked vault doesn't mean you aren't hoarding money. It is very unlikely their wealth will dissipate because enough money is in diversified investments, that they are virtually guaranteed to remain billionaires for life. With the average rate of increase of the stock market, these diversified investments will likely gain value much faster than they will ever spend it on themselves.
The most rational reason to have money is for happiness and well-being. I believe there is a principle of diminishing returns as you earn more money. When you go from $0 to $10,000, you can now afford food, drink, and some clothes and dramatically increases your quality of life. $10,000 to $20,000 now makes cheap shelter affordable, but the difference in happiness isn't as dramatic. But once you start earning over $100,000 per year, your basic healthcare and necessities all become affordable, and any further increase doesn't really impact your actual happiness that much in the long-term.
Many billionaires hoard their money even when their quality of life is virtually the same with 50 billion as with 100 million, because that money becomes a status symbol, a measure of achievement, and a goal to strive for. Some of them grew up poor, and have this insatiable desire for status and money, that they were previously deprived of. It would be healthier for billionaires to find other ways of feeling good about themselves, and give their employees a raise, lower prices for consumers, or give more to charity, since a lack of money is a big problem for most people.
