It's NOT fiat currency, it's a product that can be used to exchange funds, evade taxes, go around a country's poor economy, make a buck via day or long-term trading / investing, etc. It's an investment product; nobody uses it as fiat outside of dodgy darknet websites, and whatever webshop does accept it only does so to hold onto it for a while and hope the value increases.

There's too much inflation and uncertainty for it to be a fiat currency. I mean the value of BTC fluctuated 1000% in the span of just a couple of years.