> Now every project has a different concurrency model woven into it. Why isn't there a good solution in the stdlib for this common need?

Because every concurrency/parallelism approach has tradeoffs, and there is no one right implementation for all cases, and rusts intended primary use case is low-level and broad enough that there's not even one approach that's probably good enough for most cases.

Everything mentioned in the parent post is in the standard library.