Is that because HTML/CSS UIs are more robust and advanced than native UI frameworks? Or because native UI framekits solved all the problems that the vast cornucopia of HTML/CSS UI frameworks are still struggling to solve? Or because everybody looks at all the HTML/CSS frameworks, decides none of them solve their problem, and decides to write their own?
It's because it's so easy to create your own. HTML/CSS is very accessible, well tested by lots and lots of small communities.
There are many HTML/CSS frameworks that are doing really great job and not struggling at all. It all depends of course on what you want to do, but finding a good kit that solves your problem is definitely not the bottleneck of creating a modern HTML UI based application.