Most projects, if you say “I think the gui should look like X” they’ll reply with “show me the pull request then”. No one wants a manager when they’re already volunteering, and not enough people who have the skills to make things look aesthetic exist and contribute.
Source: look at other comments on this article.
So you only get contributions from people who can contribute on their own, which is developers.
Probably because for developers, OSS is a badge of honour and can help your CV / find paid jobs - while for designers it's just a dribble nobody cares about.
When designers partecipate, they get to contribute and even impose (sometimes questionable, think kde4) choices.
Apple is getting there though, probably because of the same managerial culture.
This is something GNOME does better for God's sake and they can't even show thumbnails in a file picker!
I'd say Windows is selectively polished. Some parts are worked out beautifully while others are hacked together. macOS is a lot better in terms of polish (but a lot worse in many other aspects).