We cannot use native widgets because of RAD. Using existing interfaces would spoiled most of U++ effectivness. And they cannot be easily encapsulated as well.
Anyway, I agree this is a problem. What we can try is to emulate them to the point where differences are not noticeable and we are ready to do so (I believe, Swing does the same thing).
Actually, we plan to be more native than usual by detecting Gnome/KDE and adapting look by running environment/theme