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.
The owners of AUR would do this for all packages they host so the inconvenience does not fall on other people. AUR has to take responsibility over the security of what they offer.