Simple example:
Go to the store page for any game on Steam and at the bottom you'll see "more like this" and a list of similar games. It helps publishers because they get free advertising off other games, it helps consumers because they can easily find games similar to ones they like, and it helps Steam sell more games. It's win-win-win for everyone. The Apple App Store has this feature. Pretty much every single movie streaming service has this feature. It is a super common and obvious feature, yet Epic lacks it. Steam also does regular events and themed sales; I don't think Epic does anything similar. Steam is simply the better salesman.
If devs or at least Epic were smart, they'd sell games for less than on Steam.
Sell on Steam for $60.
Steam takes 30%
60 * 0.7 = $42
Sell on Epic for $50
Epic takes 12%
50 * 0.88 = $44
Devs would make slightly more per sale, Epic would get more total sales, and consumers would get games for less. Instead I see Ghost of Tsushima listed for 60 USD on both Steam and Epic.