If people don’t see an issue with their system the extra step to actually install grub on MBR/UEFI can be skipped. Arch and Manjaro is not designed to keep the actual installed bootloader in-sync with package binaries. Grub normally gets installed once and every grub update is actually useless when not done properly.
Last year however there was a situation that every installation needed to reinstall grub. The Arch announcement can be found here: Arch Linux - News: Grub bootloader upgrade and configuration incompatibilities
We simply try to be very transparent about this. In the end you are the administrator of your systems.