The standard library is not versioned, so any API*/behaviour there must be maintained forever. When you put it in a separate crate, you can make better interfaces or be displaced by a better crate, while old code still compiles against the version it was written for. So even code effectively maintained within rust teams (hashbrown, rand, regex) may be in separate crates, std is specifically for OS abstraction and a shared type vocabulary.

That doesn't mean you jump to importing nonsense or trivial dependencies. The top hundred are efficient, well-designed crates of the quality you'd expect in a large std like Go or Swift, sometimes even higher as people can write better implementations that they otherwise wouldn't (or wouldn't be used over std). And those languages make breaking changes! C++ is mostly stable, so it's full of junk like <regex> or just unordered_map. Rust managed to wholesale reimplement HashMap 6 months after SwissTable released, like it is also easier to express this level of encapsulation, but that's part of countless design/interface decisions made deliberately to not constrain forwards compatibility, including a smaller std.

However, that's no excuse to have a worse developer experience in this area. We need better tools to vet and communicate the quality of a crate and its supply-chain, like community-curated or even additional org-maintained crates, and maybe a handful delivered precompiled in the default rustup distribution which can change over time. I don't think they need to be added to std itself though