There are many challenges that comes with such project. Firstly, it requires time and effort and second, it needs to adhere to the standards. These 2 things don't just happen in a span of weeks or months. Most likely there is already a plan to put a GUI library in place, but that is not their biggest concern right now.
There are many c++ library extensions created by 3rd party organizations (Boost, SFML, Qt, LLVM compilers, etc) who have taken to addressing most of the limitations of the standard C++ library. The advantage of this is that if the standard now wants to in cooperate these libraries into the standard, all that is required is a little adjustment and viola C++20 has GUI capabilities.
For example, if you look on the isocpp website that describes current projects, (https://isocpp.org/std/status),
you will see that they are also working on a Networking library, and many more. In essence, the way I see it is that c++ is trying to expand and come to the same playing field as Java and when this happens, things like GUI libraries should not even be a question.