If you are a developer you almost always eventually need some dependencies that don’t ship with the os package manager, and once some of your dependencies are upstream source, you very quickly find that some dependencies of the sources you download rely on features from newer versions of libraries. If you have multiple clients, you may also need to support both old and new versions of the same dependencies depending on who the work is for. Package managers for a Linux distribution have incompatible goals to these (except maybe nix)