Counterpoint: Windows is an Operating System. macOS is an Operating System. But Linux is not an Operating System†.

Rather, individual distributions of Linux are Operating Systems. As are the other Unices, e.g. the BSDs, Solaris/Illumos, all the old ones like IRIX, etc. And all of those, at first, built their own Desktop Environment from the ground up, just like Windows and macOS did.

As it happens, through the power of FOSS, many of these Desktop Environment projects were gradually merged, or the various distributions dropped their own DE in favor of someone else's.

But the historical path-dependence is still there, and the political lines are still mostly clear.

Don't think of KDE as "a Linux Desktop Environment." It's both not just for Linux (you can run it on FreeBSD, too!) and also isn't available for every Linux distro. KDE is still fundamentally, in some sense, "SUSE's Desktop Environment."

Same with GNOME: it might have been created by GNU, but it was mostly funded by RedHat and IBM for many years, even if Debian et al eventually picked it up as well.

Same with CDE: that's HP's desktop environment.

If you think of each of these companies as providing their own Operating System with a GUI composed solely of the graphical applications written for their own DE, the continued non-merging of these DEs makes a lot more sense. There's no more reason for them to merge with each-other than for them to merge with e.g. Android. (These OSes all happen to be based on Linux, but that doesn't really mean anything. macOS is based on BSD; should the macOS DE therefore merge with some DE that runs on BSD?)

† Defining "Operating System" here as "a software ecosystem that tries to deliver a cohesive user experience to the people who deploy it."

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The thing that makes POSIX DEs different from the ones on Windows and macOS, is that you can install multiple DEs' libraries, and then run applications from one DE "under" another DE.

Except for special one-off internal provisions for their own legacy DEs (Carbon; WoW) you can't run software written for another DE "directly" on Windows or macOS. You can only virtualize such software using e.g. Wine, or an X11 server that runs as a window under your DE.

But the fact that Linux can do this, doesn't mean that the software was designed with this in mind. There's no "Linux integrated GUI experience" that all these applications are trying to target (FreeDesktop is forum for "meet in the middle" consensus solutions to be generated when required, not a working group for creating an encompassing standard for everyone to converge toward.) There's just the individual Operating Systems' GUI experiences (plural), where a given GUI app will fit in with the OS it was written for—and maybe with any other OS that happens to share its DE as the primary DE that the system is cohered around.

tl;dr: there is no "Linux." There's just RedHat, Debian, Canonical, openSUSE, Slackware, etc. all with their own goals. And also FreeBSD, NetBSD, Illumos, and whatever wacky proprietary Unices are still in play, who all also run and support these same DEs on very different substrates, with their own even-more-different goals. Each corporate player delivers you their own experience. There's no intent, between these companies, to try to commodify that experience into the same experience. Where would these companies/projects be, if they did?