They don't have to. When you only have $100, every cent matters. Once you have 10k, dollars still matter, but cents not so much. They stop having value. And that just keeps going: once you have several hundred million, a million bucks has no value. It's not just "a small amount of money", it simply has no value anymore. You're not "spending it" so much as "whatever"ing it.

(Which is why no one should ever even be allowed to have that much money)