I think this is only true for OOP-style "widget-based" UI toolkits. I've had a very good time writing console UIs using a reactive style; obviously console UIs are simpler, but I think when people want inheritance in UIs, it's for things like 2D layout (in typical OOP toolkits, every widget type is derived from some base Widget class that does layout things) which are also present in console UIs.

I would be interested to hear rebuttals or other thoughts.