That sort of work also tends to be less well-compensated than that of SWEs which makes it more important to be paid for work (which most FOSS project cannot do).
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.
You say that you're genuinely interested, but chances are that any given maintainer is not. Willingness to actually listen to UI/UX critique is what separates the Blenders from the GIMPs.
The spacing of every component feels wrong, the font choices are off, the text is too small, there is way too much white space, the "other video" section looks like its just jammed off to the side. The overall feel of the website is very amateurish.
When I go to https://joinpeertube.org, I'm met with a double hero section that tells me about PeerTube, which is ultimately meaningless to 99% of the population.
When I get to actual content on that page, it's not an aggregate of all of the most popular videos across the federation, it's a description of the types of videos you can find on federated sites with links to the sites and not video embeds. You're telling me on the front page that I have to do legwork to find the videos I want to watch. You're already not competitive. You already lose.
Creators go where users go. If you don't build the platform to attract users, you will not get the volume of content that is required to hold people's attention. There's a reason why people come to YouTube, Reddit, Hacker News, Reels, etc. There are almost no clicks between the user and the content, and the content is plentiful.
Every time I try to use PeerTube, I have to try to find where to find the videos. As someone who's been chronically online since 1995, it's not that big of a deal. I can find them eventually. But that UX sucks, even for me, so it's probably utterly untenable for the vast majority of casual users.
You are trying to compete with slot machines by inventing a machine that makes the user read a bunch of stuff between each pull of the lever. It's not super difficult to see why that is a competitive disadvantage.