Many of the hot paths in uv involve an entirely locally cached set of distributions that need to be loaded into memory, very lightly touched/filtered, and then sunk to disk somewhere else. In those contexts, there are measurable benefits to not transforming your representation.

(I'm agnostic on whether zero-copy "matters" in every single context. If there's no complexity cost, which is what Rust's abstractions often provide, then it doesn't really hurt.)