

The preinstalled apps are not a feature of KDE (or Gnome, XFCE, etc.). Actually they all are structured in a very modular way where you can use or omit individual components. Firefox and LibreOffice are completely independent of it even; they merely add compatibility layers to make the integration more seamless.
What you experienced was something to attribute to the distribution you chose. They are the ones to decide which components to bundle and preinstall. That is also the reason why so many distributions exist in the first place, because different teams/devs have different visions about what the desktop should look and feel like after install.
AUR is the place for unverified submissions. The verified stuff typically ends up in the main repos.