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.