It's because they think being dumb makes them tough.

I don't think there are any other options - as I understand that would require coloring an icon on the fly, wether a default icon or a custom icon

Not sure if this is what you're asking for but as with Dolphin 25.04.0 I can right-click a folder, select "Properties" (not sure if that's the actual name in english; or just hit Alt+Enter), click/tap the folder icon within the modal and search for a color in the search box - lots of tinted folders (but not so much colors, though) shipped with the default Breeze icon theme

My strongly held suspicion is that it's a form of the dunning-kruger effect. People have a lot of experience using software-A so much so that they tend to overlook just how much skill and knowledge they have accumulated with that specific software. Then when they try software-B they misconstrue their lack of knowledge with that specific software as complexity.

You just answered yourself. They're just tools.

Do yourself a favor and hear that cover bit they did for Metallica's "Sad but true". They're pretty good musicians actually but they just choose to do more corny/commercial stuff – which imho is not valid reason for the hate. Sad but true.