They do not. You are free to sell your game anywhere online for less. You simply cannot use the ability to generate steam keys to sell steam games externally and then proceed to undercut steam.
You are leveraging their infrastructure for that transaction and sale, they are free to set the rules there. They do not otherwise charge you for the infrastructure costs related to purchasing the game, supplying continued redownload, supplying update infrastructure and so on.
You can otherwise take your game and sell it on the Epic store or GOG or run your own download infrastructure and foot the thousands in bandwidth bills. And charge whatever you want