Really, after paying for your mount, constant repairs, taxis, and taxes, a player has little need to spend his money. Things such as quality of life improvements (increased mount speed) or additional mounts are all optional. Especially when one considers the lack of a real player made economy. In this case, players are forced to hold onto their money, but the money itself still doesn't have any real value.
One of the characteristics of money is the medium of exchange. But in MMORPGs the exchange process is often limited. Many of the core goods, such as armor, weapons, potions, materials, are items that are easily provided by the game. Which means players have no need to use money to acquire a majority of their items and as a result they have nothing to do but to hold onto their money.