You mean they aren't? Really, how often do you have to go mess around under /etc ? A different DE is probably the biggest user-facing change you'll get.
Packaging these as different distributions sends your users red flags about compatibility and ease of use. It makes people think they have to change their computer completely if they want to try something new. It makes them think they can't have concurrent DE installs if one user prefers XFCE and another prefers KDE and another prefers Gnome. It makes them look for "Kubuntu packages" instead of "Ubuntu packages" even though the distributions are binary compatible.
There are a lot of implications of pretending like you're a whole different OS when you're just a different default DE.
And I "mess around under /etc" frequently, probably multiple times per week.
I used to live with a 40-something woman who was fairly technologically impaired, the kind of person you would expect to get a MacBook Air and call it a day. She ran Mint. Now, granted, it was installed by our room mate who was (still is, I suppose) a CS grad student, but she got by just fine with Mint, LibreOffice, and Chrome. Never used anything else, and never had a need to go plunking around on the command line.
I suspect Susan is not alone among Linux users who just wanted a cheap computer that worked and had some nerd install a user-friendly distro for them.
Don't really care, now, as I'm on Lubuntu.
I've always been annoyed at Canonical's packaging of different DEs as if they were entirely different distros.