The reason for not having a GUI package manager is simple: Arch does not provide it.
There are several AUR helpers (with and without GUI), so you can install such an app, e.g.
yay -S pamac-aur-git
Another reason is that there have been many issues with GUI package managers. So far only pacman has proven to be reliable. We support also yay which has been working well so far.
As you may know, AUR packages are provided by any user, and not by the Arch development team.That means you use them at your own risk.