Your game doesn’t sell at all because Steam adds massive value. Steam is the reason PC gaming is what it is.
Retail gets paid for a reason. Distribution has huge value. There isn’t a game out there that doesn’t make way more money paying Steam a 30% commission than they would by not taking advantage of their massive reach.
Steam taking 80% would be a much better offer for developers than Epic taking -50%. You’d still make more on Steam.