My hypothesis is that this has to do with the whitening of the UI.

Personally I love Windows XP because it's so colorful. The taskbar is blue. The start button is green. The window frame is blue, the close button is red. The sidebars are tinted yellow. Even icons like CD-roms aren't greyscale, but tinted purple instead.

Since then, people started removing color from everything. Colorful icons became monochrome, perhaps only so it could be easily switched from "light mode" to "dark mode" by switching their colors from black to white and vice-versa. Everything is now harder to see.

On Linux, most attempts to mimic retro GUIs fail because they can't tint different parts internal of a window of different colors, such as tinting only the sidebar a shade of yellow. This is rather ironic given that GTK's CSS theoretically could allow this. But in practice there is no stable public "API" for the classses used inside an application to allow users to re-style them easily with CSS. Even if I could do .sidebar { color: #ff0; }, I don't know the class name that my file manager used for its sidebar, for example, so I can't really do that.

In my opinion this the main reason modern UI's feel so bland and lifeless.