If I pay a merchant USD for some asset, they don't immediately turn around and convert the USD into something else. They hold on to it for at least a little while, and typically all entities (both businesses and people) hold on to very large sums of currency for long periods of time.

They do this because the value is stable, so they know that 1 USD today is worth the same amount (or as close as makes no difference) tomorrow, and the day after. They can hold on to the money for as long as they need, until they want to make another transaction with it.

That's radically different from how nearly all businesses treat bitcion. They never hold any BTC balance. They accept BTC as payment, but they immediately convert it into some other currency, and end up never running a BTC balance. This insulates them from the risk of BTC. Also note that their prices are nearly always based on a fiat currency and the BTC they accept is the equivalent of that currency based on the current market rate.