Mostly because gasstations decided it’s worth the effort to inform people of daily price changes.
Fuel supply and demand change a lot, so with a constant price it would have to be pretty high to prevent the risk of selling at a loss for gasstations.
With other goods supply and demand are very stable, and the effort to inform people about the daily price outweights the gains for the seller. (Imagine a grocery store would keep up some digital board with all the current daily vegetable rates instead of printing a pricetag)
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