People saying Steam doesn’t have a monopoly because other stores exist, is the same as saying Microsoft doesn’t have a monopoly on PC Gaming because Mac and Linux exist. Technically true, but ultimately meaningless because its their market power that determines a monopoly, not whether there are other niche players.
While Valve and Steam have generally been a good player, and currently do offer the best product, they still wield an ungodly amount of influence over the PC gaming market space.
Epic is chasing that because they really want what Valve has, though no doubt they plan to speedrun the enshittification process as soon as they think it safe.