Earlier quoted context omitted.
Because Archinstall is scary, even if you could train an elementary-schooler to use it. Manjaro just repackages the same thing with a harder-to-use and more confusing GUI which people are more willing to give a spin.
I first used Arch over ten years ago and nobody told me about archinstall until a few weeks ago! I was using Ubuntu out of laziness because I was tired of manually installing Arch every time I got a new machine (until now, I use Arch (again, btw)). I wonder how many Manjaro users are like me, and just don't realize that Arch ships an installer now.
It wasn't really an official option ten years ago; https://archlinux.org/news/installation-medium-with-installe... puts it in 2021 (though that's just when it was added to official install media; I assume it was available in some capacity before that)