Open source GUI development is perpetually cursed by underestimating the difficulty of the problem.

A mature high-quality GUI with support for all the features of a modern desktop UI, accessibility, support for all the display variations you encounter in the wild, high quality rendering, high performance, low overhead, etc. is a development task on par with creating a mature game engine like Unity.

Nearly all open source GUI projects get 80% of the way there and stall, not realizing that they are only 20% of the way there.

You're right, and I think that's because the core functionality of a UI lib is not too difficult. I've tinkered in that space myself, and it's a fun side project.

Then you start to think about full unicode support, right-to-left rendering, and so on. Then you start to think about properly implementing accessibility features. The necessary work increases by a magnitude. And it's not fun work. So you stall out with a bare-bones implementation.