If you were a treasure hunter and you found a million old Roman gold coins... what would YOU do with your new wealth?
Retire from an obscenely dangerous line of work. Which means its time to roll up a new character.
A surfiet of gold that does nothing but sit around actually
discourages characters from doing adventure-y things. Which is the heart of the D&D game - exploring, combat, and interaction. This is not a game of economics (not to say that economic games aren't fun, but generally D&D is not made for it).
Believe it or not, it actually gets in the way of plot more than encourages it. Which is bad for the game. It requires players to kind of gloss over what they could do with the money, because for a lot of people, it doesn't relate to things we game for. Many people want to play adventurers. And if, for most people, its not helping to acomplish said adventuring? Money is dead weight.
All this disparraging about people complaining there's nothing to spend money on is pretty sad, from my point of view. As others mentioned, even in the early editions of D&D games, we had buying strongholds and followers. There was always a
reason for all that gold. It had a purpose. Now? Its a meaningless statistic. "I get richer and lead a better life... when I'm actually in my home town and not doing stuff that we play the game to do." This is a game, and we want things for fun. And lots of people find resource management fun (thus, spell slots). There's no management here. It doesn't add anything to the game for most people.