There are lots of reasons to not want it built-in e.g. it makes the language more complex, and if bad semantics are standardised you’re stuck with them.

If the language supports implementing a feature externally then it’s a good thing, as it allows getting wide experience with the feature without saddling the language with it, and if the semantics are fine and it’s in wide-spread use, then nothing precludes adding it to the language later on.

It’s much easier to add a feature to a language than to remove it.