You fail to realize that each distro is maintained by different people. Your reasoning would make sense if the "core debian" was maintained and packaged by the same people who maintained and packaged *ubuntu.
The end user would download "core debian" from debian, and the *ubuntu "flavor" from *ubuntu. Installing debian then going "apt install kubuntu" wouldn't work because kubuntu is not in the debian repository.
If debian changed their downloadable "core debian", it could make it incompatible with what's in the kubuntu repository. They are not maintained by the same employees of "Linux inc."