I don’t know the specific motivation here but in general it’s for package development to not be tied to the language release. People also generally have different backwards compatibility expectations for the stdlib vs a regular library and that constrains the development of the package. In Python the meme is that stdlib is where packages go to die. Not all large stdlib languages feel that way. Clojure, for example, has a pretty sizeable stdlib while being code frozen without a lot of demand for change. In general, however, language developers prefer not having things in stdlib.