They're priced in sats/vb. The fiat price is irrelevant but given anyway. If fees were actually priced in fiat you would select a fiat value when you set up a transaction instead of sats/vb.
Discussion
Transaction fees are correlated to fiat price because that's what people are willing to pay. It's not strongly correlated but it is.
A 21 sat zap will remain a 21 sat zap for the time being :)
I see what you mean. I don't really pay attention to the fiat cost of on chain. My main concern is the % of the amount I'm sending that I lose to fees.