Arch doesn't come with the AUR "installed". The AUR is a repository of user scripts that exists on the internet. The user chooses to download the scripts, or install an AUR helper to download them automatically. There aren't even AUR helpers in the official Arch repos, so you need to go out of your way to install them.
Let's not take one out of Apple's playbook and limit what a user can do for "their owm safety" and because most people "don't know what's best for them".