UX work is often also a lot heavier and more subjective than the plumbing.

I might open a pull request to support some new video code, and that might only require a few dozen lines over a few files. That's easy to review, and it either works or it doesn't. Worst case they say "our convention is to register codecs as a subclass of X class, but you subclassed Y class" or something equally straightforward.

Let's say instead I wanted to change the workflow to register an account. Now I'm changing a bunch of JavaScript, CSS, templates, I'm adding pages, and I also need to update the backend. Even if someone is that into frontend work, it might take forever to even get reviewed by the maintainers because it's a massive PR.

Plus, now we've moved into subjectivity land: "I'm used to the old workflow," (because they designed it) "The last one was really easy" (for an engineer), "I think we should focus on the backend before we work on the UI," "I don't like this font because the license isn't free as in freedom" etc.

Even if you just mockup something on Figma or whatever, unless you're a maintainer it's probably going to just get ignored as a feature request. Because there's also the psychological aspect of basically being told that the UI you wrote is implicitly bad, if you're the maintainer reviewing the mockup.