Bitcoin price goes up and down all the time. If someone buys something costing 10$, the next day it may cost less or more. So it's a risk for the seller.
Why not just change the price?
You can't just put a price in Bitcoin the same as the price with fiat though it is true that it is a bit of annoying since you should be able to keep up with the price of it in the market and change the price of it in bitcoin depending on that change. It is a risk really for the reseller but in the long run, I guess it is the advantage of the seller. Another thing is that with the pandemic we are facing now, I guess shops should really accept Bitcoin.