React Native is already ported to desktops but you need to write N times for N platforms. In flutter, you can write just once after choosing a widget set, assuming you don't mind the same widgets everywhere.
So it's like Xamarin -- there's UI for iOS, UI for Windows, UI for Android, but you can also use "Xamarin Forms" and have a similar appearance everywhere. But there's nothing stopping there from being a "React Forms" or whatever that does the same thing, is there?
Flutter has MUCH better performance than Xamarin Forms though. React could theoretically have something just as good for a cross platform UI toolkit, but given that Flutter took several years to even get to a beta, I think that would probably be a long and expensive project for Facebook and they're likely just not interested in it. That sort of thing sort of runs against the core philosophy of React Native anyways.
> being a "React Forms" or whatever that does the same thing, is there?
Nothing stopping it, nope. I'm surprised there isn't something currently.
That said, it isn't so bad using different render functions for different platforms. The state management and event handling should be 100% consistent.
Besides, going all in on a single render engine limits you to creating special "holes" to utilize controls native to the platform. Im not sure if flutter supports this, but I doubt it.