This is a topic much commented upon by economists. It is called “marginal utility.” Think of it this way: A thirsty guy in the desert will give anything for a glass of water, but the same person in a swimming pool with water cooler nearby won’t pay anything.
In other words, the more you have of something, the less valuable is more of the same. This goes for food, sex, cars, rubbish money or anything else— unless the person is a bit, shall we say, “eccentric.” Of course rich people are eccentric. Thus the rule or marginal utility doesn’t apply to gluttons, nymphomaniacs, compulsive collectors or hoarders. Thus, a miser like the fictional Scrooge McDuck values every additional dollar just as much as his first lucky dime. He keeps every dollar and every coin he ever made is a huge vault in Duckberg.
But for most sensible rich people, at a certain point in life, they decide “I now am old enough and rich enough to quit. I have enough dough to retire comfortably and I won’t undergo any stress or out out too much effort to get any more.”