You fail to realize that each distro is maintained by different people.
I very well realize this. Packages are maintained by different people too!
The idea is that installing a *ubuntu would literally be the same as installing one of the many packages already available. It works for all those packages, so why wouldn't it work here?
Installing debian then going "apt install kubuntu" wouldn't work because kubuntu is not in the debian repository.
Yes, that is correct. I apologize if you misunderstood what I said. I did not mean to say that this is the current state. This is what I think how things should be.
Though for the case of kubuntu, it apparently is pretty close. You can in fact already do "apt install kubuntu-desktop", but you have to be on regular Ubuntu instead of debian. Which is fine, since Ubuntu changes a lot more about debian than just pre installed packages, so it works out for my example.
If debian changed their downloadable "core debian",
I do not suggest they should!