A complication is that AUR doesn't publish packages, it's more like FreeBSD ports in that they are the build scripts for packages and the user builds the package on their local machine. Building an AUR PKGBUILD inherently runs arbitrary code on the user machine, and that code is controlled by the maintainer of the AUR package.

So you have to scan an arbitrary bash script and determine if it pulls malware, or build the package server side in a sandbox and scan it (and some AUR scripts wrap proprietary software blobs the user is supposed to provide e.g. MATLAB, which makes those impossible to build server side). It's a very big extra layer that malware deployments can hide in.

You can only scan for known malware. Plenty of ways to write new apps to do bad things that scanners won't detect.

What does "scanning for malware" mean? As far as I know this is a totally open question, and the only credible answers (install in a sandbox) are too inconvenient for widespread adoption.