But many users don't. As far as I can tell, there is very little actual guidance about what to look for, not even to the extent of what you explain here, on the wiki. Users are told to check the PKGBUILD, and warned about AUR-helpers being dangerous, but in practice, it seems AUR-helpers are widely used, and many users likely just click through PKGBUILDs they won't be able to understand.
And, again, this attack was a relatively obvious one. Other attacks could be made much harder to notice.
Worse, distributions like CachyOS are being broadly promoted to a user base who can't be reasonably expected to check over AUR packages themselves. Unlike ArchLinux, those sometimes do seem to promote AUR-helpers. In some cases, those distributions are apparently including AUR-sourced packages in their actual repositories.
Questions about these topics often result in typical Archlinux hostility. And in some sense, that's understandable: there are other distributions that most users should be using, and the frustration of people using Archlinux who shouldn't be is wearing. It is nice to have a distribution that offers the flexibility and space for experimentation that Archlinux does. It's one of the reasons I use it on some of my machines, while at the same time recommending against most others using it.
To some extent, this is just a wide cultural difficulty with Linux, and there isn't a clear answer. On one hand, you want enough gatekeeping to keep users away from potentially dangerous systems they have no interest in understanding, and that they'll rely on without understanding in situations where they shouldn't. On the other, you don't want to keep out users who are interested in learning.