I agree with you in theory, but in practice the existing package managers are simply not suitable for the new languages (too fragmented, walled gardens, not powerful enough), so language designers really don't have any other solution except making their own manager. Ideally, though, a language's package manager would integrate nicely with the system's package manager and would know how to automatically install other packages.
Oh, I know why it keeps on happening, but it leads to a frustrating and fragmented experience. That's why it would be nice to have a common core package manager, that handles all of the tasks that any package manager will need to handle (metadata, versions, dependencies, dependency resolution, conflicts, features, fetching packages, verifying packages, managing an archive of packages, etc), plus the ability to hook in at appropriate places to provide the distro or language specific hooks necessary, and the ability to have cross-archive dependencies (including union dependencies like "either these three Debian packages, these two Fedora packages, or this Homebrew package"). That would then let you focus on solving the common problems in one place, and leave each language or distro to only have to focus on its own particular glue and policies.
