It's not like the community is devoid of good taste, just go look at r/unixporn. There are plenty of people who know how to make things look good. Why aren't their sensibilities represented in either project?
Don't get me wrong, I have plenty of issues with design choices in both GNOME and macOS (don't get me started on the "flattening" trend to eliminate UI signifiers and affordances) but on the whole it feels like they are at least thoughtful in how they choose to strike a balance between efficiency, learnability, functionality, accessibility, consistency, and aesthetics. I haven't used KDE in a while, but my impression was that it was not far behind.
Whereas the majority of r/unixporn screenshots I've seen are gaudy with a focus on aesthetics above other considerations. That may be a good taste for some and I also wonder why someone hasn't made a desktop environment for folks who like that sort of thing, but it's not surprising that the most popular ones have a more mature approach to design.
I'm not really sure what I find better. That's why the difference is uncanny. A graphic designer could probably pinpoint it. Maybe it's wasted space? Or the gray in GNOME? It looks like no one knew how big or what color to make things, and just sort of hedged.
I don't think my bias is with macOS since I've also used Windows up until 7 and never found that uncanny. It's just a different aesthetic.