I spend a lot of time on the computer, using a lot of software. This makes me acutely aware when things don’t quite work right. A button that renders incorrectly. Inconsistent padding. Unthemed dialogs. Even the best themes out there had small bugs that annoyed me. I care deeply about a smooth, consistent desktop experience.
Writing a custom GTK3 theme for my own use a couple of years back was an extremely painful process. There’s no list anywhere of the possible themable element types (I had to go through the actual source code to compile one) or the possible nonstandard options (never did manage to compile that list). I haven’t had to look at GTK4 yet, but I doubt it’s any better.
(As for how people use dark themes: put borders on things if you need to, and/or use hover options to distinguish what the active element is.)