So instead of trying to figure out which one of the dozens of GUI frameworks to use in making a window and have it change colour, you just write it using the standard library. You want to do a HTTP request, then there will be code in the standard library for that.
It will also save work trying to figure out which third party library to use when you want to do these things locally on a small test project.
Congrats, now you're stuck with something like Xwindows or MFC or AWT.
Granted AWT wasn't great but you could still make a GUI with it straight out of the box. It allowed you to make windows and buttons and start exploring the programming language.
Like I said having a standard library option won't eliminate third party libraries, it will just provide something in the box for people to start using straight away.
That guarantee is only achievable at the expense of forcing compiler developers to maintain a GUI toolkig for all platforms. Who in their right mind believes that's reasonable or desirable?
Many C++ targets don't support IO or networking, so lets not burden embedded compiler developers with standard library bloat.
Without common interfaces, flexibility in implementation is much more expensive, and innovation suffers too, as new things are harder to get off the ground without existing code that they can cheaply plug into.