The difference between Steam and Epic is that Steam gets modders who mod their Source games. These mods don’t exist outside of Valve games. Valve is paying someone who loves their games and makes content for those games. They are smart in recognizing talent and bringing it to their development teams.
Epic finds existing games with existing communities and build a wall around it so Epic becomes a gatekeeper to the fun. They stop games from working on other storefronts or pay for “exclusivity” which means stopping people from playing the game.