That's called "laziness", and is even more likely than NIH syndrome to be mitigated by experience. Certainly, having lots of coding time with a single language doesn't make you less lazy, and having lots of experience with different languages doesn't make you lazier.
The point isn't that the generalist programmer will be 100% correct in all the details in new language, it's that they'll be able to quickly (and correctly) implement the important parts in whatever language you're using. Idioms tend to be the low-order bit of a solution anyway.
Cheers
It probably took a couple of months full time in python before I had enough understanding of the std lib to know where to find everything (I sat down and read through a description of most of the functions in there).
There are still plenty of libraries that would have made my life easier if I knew about them. I'm sure there are plenty more.
Having said all that, that's just for example. When it comes to hiring I'd still pick a good experienced developer over a specific language expert for a full time position. For me it's not so much about the understanding of a specific language as it is the appetite for understanding computers / systems in general that makes truely good developers.
Ultimately the task is going to dictate the best type of hire. Short term contract, get a specific expert. Long term hire, find someone who actually enjoys developing software :)
It's something to do with how critical the library functions are to you / your system. I would never write my own compression software, but I can see why people would just to learn about the trade offs.
Third party libraries are also listed here:
http://go-lang.cat-v.org/pure-go-libs
And the ecosystem is not so broad or mature that you would miss significant libraries because there are too many options and you just didn't find them.
Otherwise you have a methodology problem. You don't do daily standups, where you'd have the opportunity to say "Yesterday, I started implementing X because I couldn't find a library" and more experienced coworkers would tell you "Use library Y instead".
Just because I've never written Erlang doesn't mean that I will automatically try to write a random-number generator (say) the first time I need one in Erlang. I have enough experience to look for a library function first.
Empirically, NIH tends to be more common in single-language developers, not less. People who place a lot of value on their "expertise" in Blub tend to do so because they're over-weighting the importance of their memory of the API details. When they don't automatically remember something, they leap to the conclusion that it doesn't exist. They're also typically a less-experienced cohort than people who have written in lots of different languages.