I think part of different distros having their own package managers is that they have their own packages. Different distros may have different preferences as to where to install things, and obviously in terms of the actual software distributed, on eg a systemd distro you want software with systemd support/that doesn’t depend on another init system. On a musl distro you want musl software. Etc. It’s just easier if you have your own package format and discourages people from installing packages meant for a different distro.