There are so many reasons. First of all, money is never enough. When you earn a lot of money and buy things that you always wanted to buy, then other more expensive things appear in life and your appetite grows too. More luxurious restaurants, better clothes, better women, luxury cruise, hotels, cars and so on. At the same time, there is a fear and the competition. You have a running business, right? So do others, so, what if someone's business becomes better than your business and your income goes down? There is a huge potential that they'll close you and if they close you, your income will go down. No businessman wants that, no rich person wants to work for others like a slave. Then everything becomes an unhealthy habit and that's why that many rich people have all the money but do not enjoy the life.
Why is "money is never enough" though? I mean I have never been a wealthy person so I do not know how it works, maybe if I became wealthy then I would want more too. But weird thing is, when I look at from where I am standing, if I have one million dollars in the bank, like cash, tomorrow, then I feel like I will never need more money.
That doesn't mean I will stop working, but I will start working for not money but for fun and do something that will probably not make money. Like I would learn painting, and paint all day, work really hard, but we all know that painting will probably not make me money. Or I will write a book, which we all know won't sell millions, so I will just be doing it as hobby. But more money? Like why would I need or want more money?