Great UX requires a lot of work that is hard but not algorithmically challenging. It requires consistency and getting many stakeholders to buy in. It requires spending lots of time on things that will never be used by more than 10-20% of people.
Windows got a proper graphics compositor (DWM) in 2006 and made it mandatory in 2012. macOS had one even earlier. Linux fought against Compiz and while Wayland feels inevitable vocal forces still complain about/argue against it. Linux has a dozen incompatible UI toolkits.
Screen readers on Linux are a mess. High contrast is a mess. Setting font size in a way that most programs respect is a mess. Consistent keyboard shortcuts are a mess.
I could go on, but these are problems that open source is not set up to solve. These are problems that are hard, annoying, not particularly fun. People generally only solve them when they are paid to, and often only when governments or large customers pass laws requiring the work to be done and threaten to not buy your product if you don't do it. But they are crucially important things to building a great, widely adopted experience.