Why does every distro need yet another package manager?
I think most package managers - the ones actually part of a distro - are old. It’s not a question of why they all use different package managers, it’s a matter of them having developed them long ago before any single one matured.
That said, there are other considerations, which is also where new ones come from - different distros will have different approaches to package formats, dependency management, tracking of installed packages and system files, some might be implemented in a specific language due to the distro’s ideology, some might work in a different way (like NixOS), and there’s probably a whole bunch that just want a different interface.
You wouldn’t ask why Linux has a different way of viewing installed programs from Windows, and in the same vein packages are not a universal aspect of Linux, so each distro has to make its own choices.
Also I like pacman, some people complain about the commands being obscure, but I feel like they’re structured in a much more logical way. Don’t confuse it with yay though, pacman doesn’t build packages, and yay is specifically a wrapper around pacman that has different commands, while adding the ability to interact with the AUR.