I remember some C++ colleagues raving on about the standard library having anything and everything they might ever need. Makes sense in a world without sane package managers and package registries, but that mindset just doesn't carry over.
A decent standard library very much makes sense even with sane package managers and registries. Just look at JS. It's awful that you need to hunt for packages for simple stuff (or implement yourself). Stdlib is usually straightforward to use, good quality, trustworthy and good and lasting support
The "good and lasting support" part is the most important IMHO. Nothing is more annoying than having to switch to another library because the one you are currently using goes unmaintained. This can happen in the "stdlib" too (e.g. PHP deprecating the mcrypt library), but it happens much less often, and typically with much more time to prepare. Also important is that, when there is a "standard" stdlib package, other dependencies you might use will probably use that - having two dependencies that use different packages for doing the same thing is also annoying.
I attribute Go's success to a rich OOTB standard library. You can build so much before you reach for third party packages. Heck, for web services, the built-in + their templating gets you pretty far.
That's a bit silly, the C++ standard library is pretty small ("the intersection of your needs, not the union" is what they say), and it's often not even really good at the few things that it does. Whether that's core stuff like hashmaps and iostreams, or more niche stuff like std::regex.
