Because the overhead and complexity is huge and useless. Unreal is not optimized to make GUI application, it's used to make games. Engines don't have anything useful to make modern app on the desktop.
In a day to day engineering perspective it’s all a mess of abstraction and indirection anyway?
If we’re going to optimize ONE stack, I’d prefer the one with all the optimized rendering and input mapping, rather than hacking that all into a bloated document parser like we’re hacking into a browser?
IMO that is textbook functional fixedness.
> If we’re going to optimize ONE stack
Why would we though? There are different classes of applications and there are multiple platforms with different conventions. Different use cases require different tools. Every time something "to rule them all" was tried, multiple niche tools grown around.
Some engines do have accessibility support. Most games don't do accessibility because it costs money. Same as web apps, by the way. It is not a technical limitation.
> What about IME?
What about it? It works just fine in any proper engine. There are a lot of non-Latin-charset speakers in the world...