Once people realize the soul devouring chthonic difficulty of doing good modern UI, they've already built on a shit foundation and been forced to realize the full system with a heap of ugly hacks and workarounds.
Then someone thinks "wow, this is way too complex! All I need is a simple UI." Then history repeats.
You can see this with immediate mode clean simple GUI library of the week. They all stagnate after getting all the basics working, and there's a reason for this.
A good mature modern UI is at least on par with a high-end game engine like Unity in terms of feature surface area and difficulty. In some ways it's worse because while the breadth and size of the problem domain is similar the problems you encounter in a game engine are probably a lot more fun. UI is a hell of incredibly hairy state management and a really long tail of edge cases.