The AUR is also not really comparable to official distribution package repositories either. Yes, it's hosted by the same people as the official repos that are comparable to what you'd get by default on other Linux distros, but it's intentionally not something that works with official package management tooling. The equivalent would be if Debian or Ubuntu hosted a repo that anyone could upload of unbuilt debian packages while explicitly not offering any tooling other than the usual tools for building a .deb from source files that you manually downloaded from anywhere else or defined yourself.

It's definitely an outlier, but more in terms of being something that doesn't really exist anywhere else, so of course the security model for it will also be an outlier. That's not an excuse for any security issues, but it's not like Arch doesn't have official repos with maintainers who are just as diligent as any other distro. They just also happen to provide a public git server that people can publish packages to with a UI for seeing some of the package metadata; people could just as easily write toolling to automate package management where it looks for a GitHub repo instead of an AUR one to fetch the build files.