Commerce in PMs isn't practical. Fiat currency is flexible enough to engage everyone in the world. There is only so much PMs and the world needs far more money than that to continue ur never ceasing expansion of humans breeding and conspicuos consumption.
Still, PMs are sort of a frame of reference and in event of societal disorder, very likely to become the currency of choice. Particularly "junk silver".
Nobody is "forced" to use Fiat currency. It's simply a practical custom.