Flutter, SwiftUI, Jetpack Compose: a whole lot of other platforms have been implementing React as their UI framework

No they are implementing Functional Reactive Programming, it isn't the same thing, even though common folks would say React.

https://wiki.haskell.org/Functional_Reactive_Programming

I wouldn't say Compose UI/Flutter/SwiftUI are "implementing React", but if your point is that other platforms have better solutions than JSX (plus all the bloat React adds to applications), I absolutely agree.