Because there is an unlimited number of things you can do with it. You're limited only by time and your imagination.
I've never understood the premise I'm responding to, it doesn't make sense, unless a person has zero ambition and zero creativity - and I don't think that's true of anyone.
I could never have enough money. I could never run out of good things to use it on. Give me $100 trillion and a thousand years, please.
You could spend $100 billion and 60 years of your life on just going after Malaria and you might not manage to vanquish it. You could spend tens of millions of dollars and decades on trying to eliminate homelessness in a small city and still not eliminate it (swap out homelessness for any number of problems that need fixing in most any nation, or city). There is what might as well be an infinite number of good uses for money, at every possible scale. I'd run out of time long before I'd run out of positive uses for large amounts of money.