Part of it is the same reason that Debian users bash Ubuntu or Mint users. Why use a derivative when the original works well?
That is a bit superficial, though. Debian isn’t ideal for everybody and neither is Arch. I’ve used Manjaro before. I like the installation process for general desktop use. Easy and straightforward. People who are used to graphical OS installers may be put off by Arch’s approach. I don’t distrohop personally and stick with Arch (btw), so OS installation is a one time headache for every device and archinstall is doable. I think the Arch installation process can be a sort of insider shibboleth for self-identifying linux badasses and a graphical install (for them) is missing the point of Arch. Whatever.
I saw a few posts on edditRay that critiqued the Manjaro team for failing to renew SSL certificates one or more times. That may have led to a perception that the team is not competent to run a distro, which seems like a very harsh position to take.
Also, Manjaro holds back updates from time to time for further testing/stability. This goes against the point of a rolling release distro to an extent, since you aren’t technically getting the very latest software. The tradeoff may make sense for you, though.
You can try copying over your dotfiles if you switch to Arch or Arco. That might be a good start for the overall layout. You might need to install any extra packages used by manjaro in addition to sway for icons, menus, power management, backgrounds, etc.