Perhaps the difference is that on mobile, users are more sensitive to bad UX because there it's so much more easy for sites and apps to be irritating/unpleasant

Perhaps the difference is that on mobile, users are more sensitive to bad UX because there it's so much more easy for sites and apps to be irritating/unpleasant to use, and big corp first party mobile apps have a tendency to optimize for ad impressions, IAP sales, or engagement over usability. This creates a market for third party apps which exist entirely to be more usable.