Because they move fiat.
Businesses operate on flow - exchange. Mining companies and refineries are paying their workers in fiat. You buy gold/silver, they are buying the next delivery with that fiat and paying their employees or making investments.
No sane business has a bank account with a large cash ballance. It is all moved immediately into some form of asset or investment.
This is why liquidity crisises are a thing - when asset values and exchanges are valued at amounts in excess of the supply of liquid capital. Currency, whether it is fiat or otherwise, is the oil of the economy.