Command line argument parsing and config loading both seem like something that the standard library should provide.
Even POSIX gives you getopt(1) and getopt(3). What other language doesn't? I can only think of Java.
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.
You're also missing the forest for the "well actually" trees: Lots of languages have argument parsing in their stdlib.
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.