They are doing it (KDE Neon) as a testbed for KDE specific development and KDE apps (including gear) and they are doing it on top of Ubuntu.
Tire manufactures test their stuff on machines that simulate cars. But they would never built their own as well they lack the expertise and why would you in the first place when all your stuff sells better when it fit on regular rims?
Same with KDE Neon, KDE is most successfully when their DE and apps are run by the most users. That's why those apps also can be compiled to non linux/Unix OSes like Windows, MacOS and Android.