when it’s the main reason why so many people use Arch Linux?
AUR is one reason why I use Arch. But not the reason. Besides AUR, Arch has many other advantages from my point of view. Like for example the wiki that also users of other distributions use. Or the many vanilla packages. Or that you can easily create your own packages through the PKGBUILD files. Or that, based on my own experience, Arch is quite problem-free to use despite the current packages.
One reason why other distributions don’t have something like AUR could be that AUR is not an official offering, so no verification is done in advance either. Thus, it has happened at least once that someone has manipulated PKGBUILD files in bad faith (lists.archlinux.org/pipermail/…/034151.html). The Wiki does not warn against the use for nothing.
However, it is much easier for the user to check the files in the AUR in advance than it is, for example, with ready-made packages in an unofficial PPA.