[Quoth]

Everyone that would likely listen to an audiobook has a smartphone. Over half of eBooks are read on phones.
An eink ereader is awkward while driving, gym, jogging, cycling, "working" etc, a phone is simple, easier to carry or have on car / bike and less obtrusive.

I've a few ereaders that do audio, terrible UI and only usable while sitting, so it would make an ereader needlessly more expensive. My PW3 does work with external USB audio but UI is dreadful and really as a poor accessibility feature than for Audio books. A phone with app is better. I have Mort for audiobooks without DRM (there is a separate Mort MP3 player, also many regular MP3 players are useless compared to cassettes for audio books, as the "place" is lost.)
I think the PW4 uses BT. Possibly because Audible is the #1 audio book service and owned by Amazon, maybe be why the PW4 has it.
Amazon wants to only stream rather than let you have a copy of an audiobook that works on anything, we are going backwards from Cassette and CD, though if you only use one device with an Audible app, you might not see the problem.