> Couldn't you do the the same thing with react native components and logic in C++? (You could) Why is this any better than just writing a react native UI? Or a flutter UI?

The tooling, that is why.

Having QtCreator, Qt Design Studio, compiling QML to native code, debugging experience.

React Native has all the gotchas from JavaScript and poor tooling for developers that never left the CLI world.

Flutter depends on Dart, a programming language that was rescued from oblivion thanks to Flutter, and is pretty much useless everywhere else.