Taxes, central bank, military, and existence of cash all preserve value of fiat.
And, further, fiat currencies aren't perfectly liquid with respect to each other.
And, further, fiat currencies aren't perfectly liquid with respect to each other.
While an existing fiat currency might derive its confidence from different sources than that of a digital currency (taxes, military, etc. vs. blockchain, mining, contracts, etc.), it does not make them that different. People still declare the coins value on that confidence/demand.