I'm pretty sure the reason for Gtk being way more common than Qt has more to do with inertia (way in the past Gtk's license was more desirable and wasn't until Qt4 that changed, so any programs before or around that time that were made in Gtk remained in Gtk even after Qt4) and language availability. Specifically for the latter, C is much easier to make bindings for and GObject provides functionality for doing it automatically, thus Gtk is available in many more languages whereas Qt's use of C++ is much harder to both make bindings and do it automatically. The end result is that Gtk is simply more widely available and so much more likely to be used by someone who doesn't want to use C or C++ (the main exception seems to be Python which has official high quality bindings for C++).
The UI designs are just a byproduct, there is nothing about Qt that makes it hard/impossible to have Gtk-like designs, but since Gtk is used by a lot of programmers, when encountering a program like those you mention, the chances are higher that it is written in Gtk than in Qt because more programmers use Gtk than Qt.
(note that this is specifically about Linux desktop applications, i think overall Qt is used more than Gtk outside Linux)
I had a stint with KDE cca 6 months ago, Gnome couldn't do fractional scaling for X11 apps on Wayland, this meant my IntelliJ was unusable (this was fixed since). So I switched for a few months - Plasma was just super glitchy, panes randomly collapse into 1px width, flicker, etc. And the UI looks thrown together, apps have Windows 9x/thrown together in Visual Basic RAD vibe.
I think OP described it pretty well - stock Gnome is actually a nice desktop environment, I prefer it to Windows, does not even come close to MacOS.
TBH i haven't used KDE Plasma much. I used to like KDE 3.5, then KDE 4 was a disaster that it took several releases after KDE 5 to stabilize (i remember trying KDE Neon, supposedly KDE's very own distro that should be rock solid for KDE - and i had severe visual glitches). But the last versions of KDE 5 and KDE 6 nowadays seems fine. Again, i'm not using it much, i mainly use Window Maker (X11) and use KDE Plasma only on my laptop as well as on my desktop when i want to try something in a more "windows-like" environment or as a different user (for default configurations, etc).
I can believe that KDE Plasma still has issues though, it seems some sort of bomb was thrown during KDE 4 times in the codebase and it still has to recover. Though if nothing else, i do use KDE apps outside of Plasma (Kate, Krita, Dolphin, Okular, Spectacle) and they all seem very solid.
I think Gtk+Gnome and Qt+Kde are being mixed here and it's creating some confusion.
After a parent said that Gnome looked better than Kde I looked it up and found a number of posts criticizing Kde design decisions (padding, etc). I fully believe the Gnome people have a better design decision. But I don't think that criticism applies to Qt itself, necessarily.
It’s not a perfect rule but I find that Qt apps tend to resemble KDE apps and have questionable design choices more often than not, even those that aren’t related to the KDE project in any way.
It’s consistent enough that it makes me wonder if maybe something about the tooling is partially to blame.
The UI designs are just a byproduct, there is nothing about Qt that makes it hard/impossible to have Gtk-like designs, but since Gtk is used by a lot of programmers, when encountering a program like those you mention, the chances are higher that it is written in Gtk than in Qt because more programmers use Gtk than Qt.
(note that this is specifically about Linux desktop applications, i think overall Qt is used more than Gtk outside Linux)