Sometimes I wonder why some things are inside the C and C++ standard libraries, and some aren’t.

As far as I can be bothered to read the actual “standards” document (that are mostly written in legalize, not in understandable English for you and me), these languages are defined against an “abstract machine”, and the actual real-world implementation of them, on computers that actually exists, should follow the behavior described for that thing, modulo some implementations details.