Mattresses
The thing is, it very much IS stuffed in their collective mattresses.
Let's look at three people, Bob, Mary, and Elon.
Bob works on a building site, earns £25k/year, and spends about £25k/year. Roughly 100% of his wages go straight back and around the economy. He's got roughly zero savings, and rents his home.
Mary works in a hospital, and earns £100k/year. She's invested a fair bit, and also has significant mortgage payments. She spends about £70k/year, or roughly 70%. The rest is mortgage & investments - probably about £500k by the end of her career. She'll spend some in retirement, and leave the rest to her kids.
Elon gets about £500 MILLION per year. He splashes his money around, buys a private island, a few business jets, that sort of stuff. Spends maybe £100 million. That's about 90% horded, not going anywhere. He's currently sitting on something like $500 BILLION of cash & shares, which he will not (and in fact cannot) ever spend. You know Tanzania? That's 6 and a half years of the GDP of the entire country, not going anywhere, not doing anything.
It's a big mattress, but it's very much stuffed, and that money isn't available to help economies grow, products move, living conditions improve, infrastructure be built, social services be run, etc.