They are of course very useful, please do not misunderstand my post. None of what I said downplays the usefulness of these efforts. I am merely suggesting that the method in which they distribute them is not efficient. Maintaining a whole different Linux distribution just to distribute a different DE configuration is overkill. It is much easier to maintain a package instead.
From the user's perspective, installing Ubuntu and doing "sudo apt install [pre-configured KDE package to your liking]" is effortless and virtually indistinguishable from just installign Kubuntu. You get the full support from Ubuntu, whereas a different distribution may not. You are not needlessly breaking away from Ubuntu.
Honestly, it could even be an install option, like Fedora and EndeavourOS do. Do you miss out on anything doing this vs an entire different distro? I dont think so.
Again, a changed DE is pretty drastic, but it does not warrant a different installation process of the whole OS or system. You should only need to take out the parts you need to, and from a user's perspective, it should be possible to make it as simple as running a command or making a choice.
I don't use arch