......... 1. sudo pacman -Sy --needed git base-devel 2. git clone https://aur.archlinux.org/yay-bin.git 3. cd yay-bin 4. makepkg -si
I think because Manjaro is not Artix, they may have different policies of which packages to include where.
Since AUR is unsupported officially, it is an understandable decision to not provide AUR helpers in any official repository at all.
But you can find prebuilt yay in some unofficial repositories (https://wiki.archlinux.org/title/unofficial_user_repositories), for example alerque, andontie-aur, arch4edu, bioarchlinux, built (currently older version), chaotic-aur, linuxrepos.
Regards!