I guess it's more about priorities, if you want to build a web app don't reinvent the underlying protocols. If you want to build an embedded system for monitoring room temperature don't invent your own locks.

But if you want to make a lock by all means make a lock, just don't go and reinvent the chip architecture...

You're taking a saying too literally.

People who take on task of writing such library, and develop it to the point it's a language-wide standard, usually know what they're doing (or learn on the job :)

Popularity of such library helps test it thoroughly on many platforms in various conditions, so there's a high chance the bugs will be spotted and fixed.

I mean, that's too literal of an interpretation.

It's more like "if you don't need to, don't invent your own [x]." People who like to invent [x] are usually smart enough to understand why that warning is there to begin with, and don't tend to argue with it.

It's not "don't invent it".

It's "be competent before you invent it, because it's hard". And if you aren't, then let someone who is do the inventing.

Don't invent anything unless your job is to invent it. Of course you might not know what your job is until you are in the middle of it.

In that case you should never run any software. Go live as a hunter-gatherer in the wilderness.