A typical KDE or GNOME system is more coherent than either Windows or (at least as I'm using it) macOS. On Windows, built-in GUIs don't even use all the same look-and-feel. On macOS, some vital third-party apps (e.g., Contexts) don't even respect dark mode in their configuration windows.
Linux has hundreds probably, maybe more.
It's not significantly better and combined with Mac and Linux a large reason many apps are using a skinned browser engine. You get better developer ergonomics for scaling, languages, screen readers, general accessibility and portability over many dedicated UI component libraries.
MAUI could have seen massive adoption with a Linux target (even if just gtk). It's not the biggest in terms of users, but it's become very large in terms of developer mindshare. Without it, it's an also ran.
I'm not a fan of gtk or QT. I don't really care for the shortcomings of most options at this point. Everything kinda sucks. Skia may gain traction and patch the missing pieces, but who knows.
Right now, when is the only mostly safe option for cross platform general desktop apps. Which is weird.
In terms of fragmentation, gnome and kde are the vast majority of configurations. Even other DE are likely using the underlying graphics library.
Given the level of customization that System 76 has to do with gnome for every release, it's probably easier for them to just create something more congestive for their designs.
And given that they actually sell and support hardware, are far more likely to make better choices for more general users than a lot of other DE in my opinion.
I do wish they'd make a deal with framework to get it with Pop as an in the box option supported by System 76... I'd pay a bit extra for that.