[tyingq]

>> You can respond to events, like user interaction, by telling the framework to replace a widget in the hierarchy with another widget. The framework then compares the new and old widgets and efficiently updates the user interface.

>No explanation for this odd design is provided.

I suspect that's to support how they provided both an iOS and Material design look and feel. For example, there isn't a single button widget with different styling. There's a CupertinoButton and a RaisedButton.