Maybe you should re-examine your priors, because reality is sending you a lot of data that refutes them. If it were feasible to do this, I think it would be done by now. There are a lot of reasons it is very difficult, and while there may be no reason it is impossible, something can become practically impossible when you pile constraint after constraint on it.

One example - wxWidgets exist, and wxDart exist, but getting to something half as usable as Flutter from there is almost impossible. I defy you to even build a layout engine that works on all three platforms. After that, you still have to find answers for theming, spacing, responsive behavior, component composition, inspection/debugging, hot reload, accessibility conventions and on and on.