I know :) , and I searched for that too when I joined a QML project after working on a React one. So far there's nothing like that, thus the "around" in my comment.
Now, about why no one tried that yet, I'd propose that newcomers (including me) want it but don't know the platform well enough to build it, and experienced C++/Qt-ers could try it, but they don't care about new frontend stuff like React the slightest bit, as they are used to their own thing (which works well too!).
It's just two different technical worlds with different working solutions, and little interest to pick ideas from the other side of the fence. That being said, I'd love to be proven wrong.
Not exactly the same as React render(), I have started an experimental project to update QML UI by diff:
QSyncable - Synchronize data between models
https://github.com/benlau/qsyncable
p.s In fact, Qt Scene Graph is somehow similar to React. Both of them will only apply the changes to screen. React manipulates the DOM, but Qt manipulates the OpenGL render pipeline.
The QuickFlux is promising start for the Flux pattern in Qt.
I was thinking more on the lines of React Native [1] like support in Qt.
Essentially, having React render() function backed by Qt Quick Controls?
[1] https://facebook.github.io/react-native/