The excuse we all use: people were still new to that kind of devices and apps and having UIs resembling the real things (i.e. the Notes app, looking like a real notepad) was a way to ease the transfer from a real life experience to a digital one. And this actually makes a lot of sense.
Your analogy of comparing Conversational and VUI in this manner makes sense. Also it is true that skeuomorphism was highly deriving from metaphors, just the way the original Start OS drew on the Desktop metaphor. Interestingly, this is a metaphor we have been clinging on to ever since. Metaphors are not bad in fact and they serve as cognitive aids — which is why the old iPhone ui looked so much like the real world. This trend away from Skeuomorphism is a natural Hegelian shift in my opinion, I write about that here:
We will see the same thing happening in different facets of life. It does not mean, in my opinion, that one approach is better than the other. It is just better at that moment of time…