The dirt you live on is managed by a government. It sends you a bill for taxes, denominated in the fiat currency. If you do not pay that bill, they send men with guns to either take it or arrest you.

That is the ultimate source of a fiat currency's legitimacy. Everything else flows down from that: the government pays for its own goods and services in that currency, and everybody else uses it for transactions because it's the lowest common denominator. The government manages its currency through a bank to try to keep its economy on a good footing.

Any other currency lacks that fundamental "I have to have it if I want to live here." It might have other advantages, but in the end you'll have to convert it back to the currency accepted by the people who manage the dirt you live on. The "fiat" of the fiat currency is a government order, backed by real-world power. A non-fiat currency lacks even that much.

If my store refuse fiat, then the police may come

If my (oil) nation refuse their fiat, then their military may come

Sounds like pretty convincing arguments to me to "believe" in said fiat