In the early days you would pick a window manager: twm, fvwm, icewm or whatever. But while it might make your desktop look superficially pretty, it wouldn't affect any of the applications you ran, which would probably still be an inconsistent mess of different toolkits with different looks and behaviour.
Then when KDE first started it set out to address this: there would be a set of applications and tools, with consistent behaviour, and good support for writing new applications with the same framework. It was no longer correct to refer to KDE or GNOME as a 'window manager' - they were much more.
Has freedesktop.org taken over the job of library development to such an extent that GNOME and KDE can be considered as mere window managers?