I struggle to see the reason why web pages and apps are even two different things.
The browser is really just a cross-platform platform for building applications.
The browser is really just a cross-platform platform for building applications.
There should be a singlular framework for creating desktop apps.
You'd never design from the start the bonkers HTML/CSS, box model, and other junk we are stuck with, but browsers have a number of good aspects that you'd probably end up with in a complete redesign.
The problem is multifaceted. UI toolkits are very hard to do well (I have a lot of opinions on this, and my own crossplatform toolkit), even on a single platform. Just that alone is huge. There are basically no overall great UI toolkits, even if some have good parts. Even a theoretical great UI toolkit will have quite a task to achieve parity with what browser UIs can do.
If we could get past that (spoiler: we can't, no way in hell) then we'd also have to be willing to ditch all the legacy browser stuff to switch billions of users and devices to something new AND we'd still have many other big problems to deal with: getting the committee/everyone to agree without ruining it, security, privacy, etc.
We've accepted we can't really fix it, so the only thing we can do is keep extending it. That's how we got here.
WPF was an amazing UI toolkit. Heck Silverlight was a great UI toolkit.
Even Swing was nice to code in, it just ran horrible on machines of the time.
Eg Swing has all those problems, in addition to ugly themes. The uncanny valley resulting from trying to mimic native UI was bad. At least nowadays users don't necessarily expect native looking UI, even browsers don't do it.
It's not a question of can you make a nice UI with it. You can drive a nail with a rock. The high pain needed to be productive with bad tools leads to Stockholm syndrome. People would rather stick with what they know than go through such pain again to learn something new, and rightly so when the new one is likely just as bad in new ways.
Also now nothing resembles native platform UIs, so that isn't a big deal. Swing was very theme-able, which is all anyone cares about now days.
Swing was themeable but good luck if you ever try to do it. Customizing an existing theme is too hard. All components have opaque theme objects that are part of the complexity problem I mentioned and often hide functionality and parts you want to customize.
It's easy to shit on everything, but I really think UI could be done much better and that most or all toolkits make big mistakes that hurt productivity and cause a lot of suffering.
Regardling difficulty: HTML/CSS is essentially already cross platform UI toolkit. Yes its hard, but Im saying if one were to make it, I dont see why the same UI framework shouldnt work for a desktop application.
Building the web toolkit took enormous effort and it sucks in many ways (see article). Theoretically a new one could be created and could also work for desktop, but there are lots of reasons why that won't happen.
Why?
We have seen all these framework claimed to be cross platforms and none of them are not able to make it to the top framework being used.
Web is the only "platforms" with some kind of success by abstract everything away so most of the browser display roughly the same things to all users on all OS.
Implement the UI framework at an OS level. The default OS-specific design should live on top of that. If you were to build an OS of the future, there shouldn't be this need for this distinction between web and desktop.