You could say all those things about react native or other frameworks.
Like any reason why Airbnb can’t open source their solution? Seems battle tested.
The thing is that many people HAVE solved this.
Your correct this is hard, but I have to disagree that it involves painful code. Declarative UI in all 3 clients is a major enabler for doing this in a nice way.
Some context - https://github.com/MobileNativeFoundation/discussions/discus...
(Edit to add context)
It hasn't emerged because nobody has done it. Nobody has done it because it's unbelievably hard and involves a lot of painful code of the sort that is not fun and therefore that people must be paid to write. Nobody has been paid to write it because nobody pays for dev tools and it's not in the best interest of any platform vendor to commoditize platforms.