Because the standard library is where modules go to die. Slaving the release cycle of a library to the release cycle of the language is bad for the health of that library, especially in the case of javascript where code often has to be written to run on old browsers with old versions of the standard library.

Now having a metalibrary that depends on a curated collection of popular/useful utilities is probably a good idea - but isn't that exactly what many of the libraries that broke due to depending on left-pad were?