Because Manjaro isnt exactly as it seems. Sometimes it's the worst of both worlds. I wont deny that it is convenient but (Rant alert) : The idea of delay Arch packages for testing seems nice at first, until you take in the fact Manjaro will have to push some of the packages out before that period for e.g: security reasons, while some might take longer. The thing is, upstream Arch repo is designed to work together only on the packages upstream version, and as per Archwiki said, partial upgrade is highly unrecommended, and it is not uncommon to update your manjarobox and everything went smoothly, until you reboot and fallen into a dependency hell. Sometimes it can cause serious security issues. And with that in mind, the AUR also works with the assumption that you are on Arch's upstream packages, not Manjaro one, and well, dont need to tell you how it can cause problems down the line.

If you want a good convenient no "nerd" fuss distros, I recommend Pop!OS, Mint, or even Debian. If you really want to use AUR (trust me it's not as special as most expect, you generally only want to go there when you must) and really don't wanna use Arch, there's projects like Antergos, Artix,... that have much more sane approach.

I acknowledge tho that I 100% believe Manjaro users can get perfectly stable experience, and these things I mentioned had never be inside their scope. It's just you can get very similar experience with better management even in Arch-derived space, so why not go for those instead?

It's the latest craze among the DIY Linux crowd. Like what gentoo used to be. It has a very opinionated fanbase so they will advertise whenever they see an opening. Hence the "I use arch "btw meme