Go is being developed by Google and as such is designed to meet their needs. Google's view of package management is to bring every package in-house and manage it themselves. So, if a Go program needs package "A" then Google will directly import that package into their workflow.

This is actually one of the reasons that I don't think Go is a very good language for most people/organizations. It was conceived with a specific set of guidelines that Google needed; easy to learn, performant, etc. Go is also designed to be used by teams of thousands, so it's much easier to adopt misc packages into the fold and maintain them than it is to manage links to outside requirements.