Go is still a young language
Once upon a time, young languages showed their youth by having core incapabilities (important features not implemented, important platforms not supported, or the like). This is no longer really the case today; now languages generally show their youth through limitations in their standard library. The reality is that a standard library that deals with the world of the modern Internet is both a lot of work and the expression of a lot of (painful) experience with corner cases, how specifications work out in practice, and so on. This means that such a library takes time, time to write everything and then time to find all of the corner cases. When (and while) the language is young, its standard library will inevitably have omissions, partial implementations, and rough corners.
Go is a young language. Go 1.0 was only released two years ago, which is not really much time as these things go. It's unsurprising that even today portions of the standard library are under active development (I mostly notice the net packages because that's what I primarily use) and keep gaining additional important features in successive Go releases.
Because I've come around to this view, I now mostly don't get irritated when I run across deficiencies in the corners of Go's standard packages. Such deficiencies are the inevitable consequence of using a young language, and while they're obvious to me that's because I'm immersed in the particular area that exposes them. I can't expect authors of standard libraries to know everything or to put their package to the same use that I am time. And time will cure most injuries here.
(Sometimes the omissions are deliberate and done for good reason, or so I've read. I'm not going to cite my primary example yet until I've done some more research about its state.)
This does mean that development in Go can sometimes require a certain sort of self-sufficiency and willingness to either go diving into the source of standard packages or deliberately find packages that duplicate the functionality you need but without the limitations you're running into. Some times this may mean duplicating some amount of functionality yourself, even if it seems annoying to have to do it at the time.
(Not mentioning specific issues in, say, the net packages is entirely deliberate. This entry is a general thought, not a gripe session. In fact I've deliberately written this entry as a note to myself instead of writing another irritated grump, because the world does not particularly need another irritated grump about an obscure corner of any standard Go package.)