It sounds like you are speaking about the deficiencies of some particular game, this certainly isn't true of all MMOs. GW2 and Eve both support buy orders, for instance. Items that drive the economy are also not always obtained incidentally, often they must be crafted by someone with high enough skill using rare components.
To be fair, EVE economy is kind of an exception in the world of MMOs... Since it's actually the central part of the game.
Given that the In-Game currency is purchasable with fiat currency, wouldn't it be reasonable to surmise that hyperinflation doesn't occur in the same way due to the fiat anchor for IG currency values?
Only if it goes both ways. The ability to purchase in-game currency with fiat currency actually makes inflation worse by introducing even more money to the in-game economy. Most games currently do not allow in-game currency to be "legally" converted back to real world money directly. Their solution is the ability to convert large amount of in-game currency to meaningful metagame items. In Eve, you can use in-game currency to pay for your subscription. In GW2, you can use in-game currency to pay for "gems", which are used for microtransactions such as server transfers, rare skins, additional bank storage, etc. These gold sinks serve as an important valve to control inflation of the economy by permanently removing that currency from the game.