We don't, but it is the easiest way to ensure the package manager is adapted to the language and the library ecosystem. It is also easier to add features, remove misfeatures, and fix bugs when the package manager only has to handle a single language.

Once we understand package management better -- we don't understand it well yet -- it will likely be feasible to consolidate many package managers into a few omnimanagers.

In theory nix can solve that, in practice is more ergonomic to leverage a language's native package manager even in nix.

Need is the wrong word, sometimes it is necessary. Look at npm, long time they not include new features and made less progress. That time yarn was born. pnpm and other followed, cause npm did not have features, security solutions, and other things that people want.

In theory we need only one for all, but in reality, that is impossible.