First, Go had package management – it just has been optimized for monorepos.

Second, since early days of Go, they said that acknowledge their Google monorepo bias, and can't implement package manager that works for everybody without understanding what people outside of Google need. They consciously gave time for community to grow and mature and get enough feedback before implementing proper solution, while actively studying package management solutions from other languages.

I definitely can't call it "refusing to learn from established work". Of course, they could've just copy existing suboptimal solutions without properly giving it a thought – that's what most of the languages do, after all – but that's where Go choses another path.

because implementing package manager well is not a trivial work. I had no problems with vendor (godep), later I used Dep and now modules. There are still some problems with module handling for vscode (gopls) but that will be sorted out over time. If you don't like the way Go is going, what stops you from using Java with maven, gradle or whatever...