> which is really integrated into a programming language or part of the standard library.

Not a programming language, but the programming language: C. The toolkit needs to be available as a C API because that lets it a) provide stable API and ABI and b) provide bindings for multiple other languages without having to jump through hoops, especially for other compiled languages (binding Qt to Python might be easy, but bindings to something like, e.g. Free Pascal requires an intermediate C++ library that exposes a C API that itself can be used from Free Pascal - and applications need to distribute it that library too).

Unfortunately the vast majority of GUI toolkits are not writtne in C but in C++ or some other language that makes using them from anything than the developers' favorite language a pain. And really the only mainstream that is written in C is GTK which has a complete disregard for proper backwards compatibility.

(you may think that a library only needs to expose a C API but it can be written in any language, however for something that doesn't have any widespread availability, you may want to link to it statically - however that can be an issue with anything outside C/C++ - as an example i recently tried to make a FLTK backend[0] for Lazarus since FLTK is a C++ library that the devs encourage to link it statically and it would allow creating GUI programs that are self-contained binaries... but statically linking a C++ library -for which i had to first make a C wrapper- in a non-C/C++ turns out to be a PITA under Linux if you are not g++ as that does passes a bunch of magic flags to the linker and impossible under Windows - or at least msys2, so i gave up).

[0] https://i.imgur.com/W6XbLkr.png