In the Linux desktop world, there is no single entity in control over the entire software stack ranging from the kernel all the way up to the desktop environment. In a typical Linux distribution, you have the Linux kernel (run by Linus Torvalds), various command-line tools written by many different developers and managed by different projects (some of them are part of the GNU Project, but others aren't), some type of display system (this used to be solely X11, but Wayland is growing in popularity these days), one or more GUI toolkits (Qt, GTK, some custom ones), and a desktop environment (typically KDE or GNOME, but others exist). The goal of a Linux distribution is to take these disparate parts and present a coherent system to the user.
The problem, though, is that because the Linux desktop is made up of disparate parts from separate teams that have separate, often competing visions for their roles in the Linux ecosystem, often major changes are made with little regard to how they affect the system as a whole. This is the essence of the lack of control over the entire software stack. Thus, the developers of X11/Wayland, Qt, GTK, and other infrastructure can make breaking changes, and application developers relying on those subsystems have to either adapt or lobby for forks. Thus, the churn.
By comparison, Microsoft is in full control over Windows, and Apple is in full control over macOS. Even the BSDs are in full control over their base systems (for example, OpenBSD isn't just a kernel; the OpenBSD team also has control over the command-line tools that make up the base system), though I'm not aware of any BSD (besides macOS) that is in full control over GUI environments. It's not to say there is no churn in these environments; indeed, macOS does not prioritize backwards compatibility like Windows does and thus there's some churn macOS developers need to deal with in order to keep up with the latest releases. But there seems to be a lot of churn at the GUI level in the Linux desktop ecosystem.