Because the AUR is a pretty low quality repo. Not sure if anything has changed since 2 years ago, but last I used arch, the AUR was full of broken, abandoned, and unbuildable packages. The Debian repos, fedora+rpmfusion, etc, provide a comparable number of software packages with substantially higher quality, hence no need for the AUR. Fedora actually has COPRs which suffer from the same quality issues as the AUR for similar reasons.
Thing is, the AUR isn't really meant to be your primary repo. You can really get anything into the AUR.
The reason why I love it so much is because if I need a package that's not in the main arch repo (which tbh isn't many), then I don't need to bother going to some github page and compiling from source, I can just find it in the AUR and it's all done for me. I did this with things like goverlay and it's one thing that I immediately miss when I distro hop away from something arch-based.