Maybe for AAA devs. I’m thinking of indie devs as well, who may not be thinking of “big business extract max profit no matter what” optimizations.

You also seem to be leaving out of the equation that some customers will buy a game for $15 but not for $20. If selling at $15 on Steam would give too little money, but selling for $15 on Epic would give enough money, it doesn’t seem absurd selling at $20 on Steam (with the biggest market) and for $15 on Epic (with the higher margin) to reach a few extra customers with the lower price point.

The reason games go on sale so often is because lower prices can mean more sales. Lower middleman cuts can allow for lower prices.