I find it odd that a pen is used in the icon for a digital word processor. Design experts and developers; why do you think Apple uses the pen icon for a product that isn’t designed for the pen (yes I know that one can use the Apple Pencil to annotate/draw in Pages but that is a tertiary use).
Thoughts?
I am not a design expert, but my personal thoughts.
I believe because they market it as writing and word processor software than typing software. It is meant to replace writing on paper. This is not the only place where writing is represented by pen. Type the word writing, and iOS will suggest this emoji 
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I also believe our minds will connect a pen icon to writing more than any other design.
That is my assumption as well but its interesting that another digital writing app, iA Writer, uses an icon that clearly communicate writing but in digital form.
Obviously, there is no right or wrong answer; I just find it an interesting choice–probably because I’ve been paperless for at least 10 years. The only time I use a pen is to sign documents and I try to do that digitally whenever possible and permitted. 
Hmmm … the icon for Pages clearly indicates writing to me.
iA Writer looks like the ‘A’ ran into a wall. It does not communicate writing to me at all. 
Nostalgia? It’s probably a leftover from Apple’s skeuomorphic days. And it will probably disappear in a few more years, just like this:
That made me laugh. 
I see letters typed next to a cursor. A bit like an Ink Blot test.
Diverging comment here… I like analog clocks and watches. I find them far more useful than digital clocks. For example, if I have a meeting, I don’t “need” to know that it’s exactly 10:30am but I just need to know that it’s “close” to 10:30. Additionally, the position of the minute hand shows me the visual “space” of how much time I have left to a particular time.
I also like sparklines. 
I do as well but I also like the benefits of digital and health tracking. My compromise is using two watch faces on Apple Watch.
Digital
I totally agree, I use the Simple analog watch face on my Apple Watch. 
We have the same issue across all of iconography basically - it’s rooted in the industrial revoloution more than 100 years ago. Just look at corporate presentations, where “Technology” is often depicted with mechanical cog wheels, the “Idea” icon is still Edison’s lightbulb, “Industry” is that old factory with three slanted roofs and a smoke stack. There are so many more examples when you start browsing any icon pack or icons bundled with PowerPoint or Keynote.
Of course, creating new and representative icons is a big challenge, as most of what we do is confined to some version of a rectangular screen shape.
A possible corollary is the road sign for a rail crossing. In most (all?) countries it depicts a steam train, despite such vehicles being very rare on most such crossings. Why is it a steam train? Because it is instantly recognisable as a train to a huge proportion of road users.
Now, you’re not in danger launching your apps as you are approaching a crossing, but the instant familiarity is, I’m sure, what they’re going for.
What is that? 
What Russia looks like in Putin’s head?