Manjaro does not maintain the AUR. The AUR is an independent repository with build scripts uploaded by Arch users. Manjaro offers access to the AUR — for users who know what they are doing — via pamac and yay, but that’s where our support ends.
Those should normally be included in the base-devel package. The base-devel package is however not installed by default.
I my opionion, if you provide an installer for AUR packages then this installer should deliver all dependencies that are neccessary for basic usage of AUR packages.
You can create your own distribution then and provide that. I would go as far as making an exam about understanding of AUR you have to take before you even get an option to build anything.
The pamac package manager offers support for accessing the AUR, but it’s not enabled by default, and there is a reason for that. Manjaro has no control over the AUR, any more than that we’d have control over Snaps, FlatPaks or AppImages. But pamac can access those repositories too, if enabled.
Therefore, Manjaro does not recommend the use of those repositories unless one knows what one is doing. Needing to install the base-devel package is considered “knowing what you’re doing”.
I generally agree that such things can install automatically (like toggling snap for example installs the snap service). But manjaro is a small team distro, only one man codes pamac so this is a low priority task.
But there is also a target conflict here: the more one click the experience, the less tech savvy users will use the distro. And no matter how one-clicky it becomes, an arch based rolling distro will never be really suitable for absolute beginners. So at the end of the day, there will just be more complaints. We already see this in other areas of the OS: the gui for kernel install clearly states what is recommended, yet people install non-lts kernels and forget to remove them in 3 months. Or even RT kernels, just because they have a higher number. If they had to use the command line, some of these people would not have bothered to install unsuitable kernel without reading what it is and would have stuck with the defaults.