Because Flutter is based on low level drawing using the Skia library - all components are created from ground up. That means working for the web is a case of translating these low level drawing commands to canvas, so the solution can be whole and sound.

RN on the other hand wraps existing native components for that platform, thus it doesn't have any control over their drawing and can only adjust them via the public interface. So converting that to web is just an approximation "The user wants a radio button, I'll create one from scratch for the web". Furthermore it doesn't provide low level drawing, the closest you can get is 3rd part packages like `react-native-svg` which don't translate their code to canvas either, though if someone wants the large & intense task of adding it to that I suppose they could at least.