It's not closely tied to real money; not by design. That's a side effect, due to the relative stability of the in-game currency, and the ability to convert in-g

It's not closely tied to real money; not by design. That's a side effect, due to the relative stability of the in-game currency, and the ability to convert in-game currency into in-game stuff, and the number of players. This creates a real-world demand for in-game stuff, and bingo, you have a market.