Why? It is a convenience - not a necessity.
It is not part of the basic manjaro-system. It is included if the maintainer of the profile deems it useful.
You can check the profile - it is not included.
You can easily install it if you like
sudo pacman -Syu bash-completion
I don’t even use bash - I use zsh with a custom theme - but the manjaro zsh configs are really - really good.
If you run kmscon - you can even get that fancy theme working on the console too.