Ok, now, what languages beside Python and Go provide Command line argument parsing? And Go doesn't do that in a `professional` way. You either write your own, which can easily turn into a clusterfuck or use a third party library. Even in Go, people use cobra[1]. Also embedding a lot of functionality in a standard library isn't great as well, because if some vulnerability is found, it's really hard to patch it, because you need to push versions and (for example on Linux) some distro maintainers won't push it for `stability` etc. A standard library should provide basic functionality (in most general areas), but not very advanced one.
POSIX is not ISO C or C++ standard. On Windows, what are you gonna do?
Also, other languages are Rust, Kotlin, Swift (to name a few `modern` ones). Yes, Kotlin and Swift have `first class` CLI parsing libraries, but they are not part of standard library.
"First-party" is distinct from "first class". The difference between a first-party library and the standard library ranges from "slightly weaker compatibility guarantees" to "it's supported in all environments where it makes sense, but the language can run unhosted so that's not everywhere" to "no difference at all, we just didn't want to package it with the compiler".
You're also missing the forest for the "well actually" trees: Lots of languages have argument parsing in their stdlib.
It's not part of the standard library, but Swift has the first-party ArgumentParser[0]. Other languages could use a similar model (though what "first party" means for JavaScript is unclear).
as if there even was one command-line or config standard. especially across different operating systems.
it absolutely does not belong in stdlibs, where it can never be changed. that's how you end up with too many terrible CLI tools using Go's `flags` package.
[1] https://github.com/spf13/cobra