GUI is definitely a big reason, but features as well. Epic could make their store snappy as hell and their GUI easier to use, but that doesn’t give us controller remapping, game recording, an overlay with a web browser, per game notes, per game news, community guides, baked in mod support, performance data overlay, controller centric mode (big screen mode), and god knows theres more I’m missing.

Personally, I’d put my money on GOG becoming the real competitor over time. GoG Galaxy feels absolutely better than Epic does, and the fact it integrates other storefronts is such a major benefit that it makes it at least capable of competing with steam in the long haul. If GoG Galaxy gained all the features Playnite has, it would easily be able to compete with Steam.

Steam’s “monopoly” is purely because none of their competitors bother with pro consumer updates.