Irrelevant, as I didn't say the Go team should build a package manager, just that Go lacks one.
And I mean a dominant one -- there's a few attempts for Golang. Default doesn't have to mean "core-team built". All the package managers I've mentioned are de-facto standards for their respective languages.
Go explicitly dismisses everything learned by the programming language (and tooling) community at large because its authors "know better".
There really isn't much to say about Go anymore in my opinion, other than answering the title of this post with "Don't".