Fiat currency is backed by the existence of an economy that exchanges fiat currency for goods and services. If that economy disappears you get hyperinflation. See Zimbabwe and Argentina as examples of countries that destroyed their economies and suffer from hyperinflation as a result of massively reducing the ability to exchange money for food.

Think about it this way. You play a video game with micro transactions. You receive 1000 units of premium currency. Why is that currency valuable? It's valuable because you can exchange it for cash shop items. Imagine if the cash shop shuts down but the game keeps running. Your currency would be worthless because you lost the ability to exchange it.