What happens is this:

Arch Linux says the AUR is just a collection of user scripts. Use at your own risk. Anybody can upload some shit. Always check the PKGBUILD before you continue. Never use so-called AUR helpers to automate the process to the point that you have less control, also wrt future updates and upgrades.

Then some Arch-based distros create AUR helpers and integrate them into their distro experience, even with automatic updates & upgrades and GUIs and whatnot. Some of these distros are very popular, even more so than Arch Linux itself, in the short term. This also contributes to the pollution of the AUR. Malicious hackers are never attracted to a less popular distro that requires its users to understand what they're doing.

Blame those distros and all who contribute to AUR helpers, or tend to not read the PKGBUILD before installing - not the AUR itself.