You’re probably right, but it may be too early to put a firewall in place for everyone from the time of installation. manjaro is a small distribution, and you probably guessed that introducing a firewall that is switched on by default requires a lot of support at first.
All installation routines for packages with network functions would have to be adapted so that they open the necessary ports in the firewall (after consultation?). This can cause a lot of trouble over a long period of time. I don’t think manjaro can play the pioneering role there.
Under Linux it is the case at the moment that everyone has to take responsibility for their own system. Instructions are given sufficiently e.g. in the ARCH-wiki. There are instructions for firewall as well as for backup (is also important and not set up automatically). For system maintenance …
If you want to secure your system, here are a few keywords that are worth looking for: ufw wireguard btrfs-send opensnitch … the list should be longer, but I know only so little. 