If you build a cross-platform UI framework, it'll probably end up looking a bit ugly and custom on every platform (eg Java Swing). Making a cross-platform UI toolkit that looks native everywhere is an insane amount of work, because different platforms have very different UI toolkits, with different built-in components and different platform conventions. This problem becomes 10x harder if you want to make it work on mobile as well.

Some people try anyway - and bless their cotton socks. But electron (and arguably Qt) are the closest we've got. And even then, most electron apps seem to be full of custom components.

Problem is, the multitude of platforms, screen sizes, capability sets etc. that we must support in practice make a very compelling case for a unified runtime, which ended up being the current web tech turducken clusterfuck we have today.

If the CPUs were slower, hovewer... one can only wish.

The VB6/Delphi era (called at the time RAD for Rapid Application Development) was oriented around proprietary UI and component frameworks. They were planned out a long way in advance and solved a lot of problems up front, but, they were tied to Windows and then you had to buy the dev tools on top.

These days people usually want to write web apps if they aren't doing mobile. The web as a UI framework is basically evolved rather than designed. The features you need to support RAD don't exist. Moreover the web UI community would view them as bad practice!

RAD tools were typified by a few things:

1. A standard way to define components that is statically reflectable (so you can populate property and event editors), and get documentation on the fly, etc.

2. MVC: a standard way to separate code from layout.

3. Layout management that's integrated with the component hierarchy.

4. A standard toolbox of high quality, high effort components.

5. A marketplace for components (which means they need to be distributable in binary form).

The web has none of this and isn't going to any time soon. For example, there's no agreement on how components work. React has one or more different solutions, Web Components is a different one, Vue yet another. MVC has been abandoned: React doesn't know anything about that. Code and layout are intermingled together. JavaScript is dynamically typed, largely reflection-free language. You can't automatically work out what the props are for a React component short of parsing the source code. You can't easily distribute proprietary components either because there's no way to enforce licensing (so a lot of users wouldn't pay, in reality).

And then there's the components. Browsers don't provide good enough components out of the box, unlike Windows-oriented RAD tools, so devs have to invent their own. Then React makes things worse by conflating theming with the component library. The result is that every web app has its own unique set of widgets with its own unique set of limitations and bugs, very few of which ever reach a high quality level. Widget development is expensive, which is why RAD tooling centralized it and supported a marketplace, so devs could amortize out the cost of writing widgets across many projects. The whole React/Jetpack Compose approach basically spurns all of that and says if you want to change the way your app looks you get to redevelop the widgets from scratch. And everyone does want that, so there are floods of duplicated work that's open sourced and then abandoned, but nothing ever gets critical mass.