The Epic First Run programme allows developers of any size to claim 100% of revenue if they agree to make their game exclusive on the Epic Games Store for six months.
After the six months are up, the game will revert to the standard Epic Games Store revenue split of 88% for the developer and 12% for Epic Games.
Well 0% fee for a store that has zero added features vs 30% for a launcher with cloud saves, overlay, online couch play, tradeable in-game items, gifting, community, profiles, wishlist, notifications, etc.
Someone has to pay for the server time and storage.
Oh and Steam has way bigger userbase.