[Rusky]

System-wide package managers give you a single, global version of a package built to interact solely with the rest of that one distribution.

I absolutely do want to control how dependencies are built- I want to build them in debug mode, link them using LTO, cross-compile them, or (Rust-specific, since someone mentioned Cargo) switch the panic strategy. Further, many packages I depend on via language-specific tools likely will never be in any distro's repository, nor should they be.

As for assuming a build environment, that may be an issue with a language like C or C++ that has already gone a different route, but it's better when a language has a single, established way of building things that a package manager can integrate with.