From my experience it is obviously the latter. Reading well, on paper or on screen, really requires you to put your complete attention to it. Audio (podcasts) and video (youtube) have the advantage of not requiring your complete attention. Everything else follows from that. Of course it can fit some people better. Just not where it matters.

There’s no such thing as multitasking. It is a literal illusion and is one big reason why people who can’t sit down and actually read a book (or lie down with eyes closed and LISTEN to a podcast/lecture) produce for themselves the illusion of understanding.

And then you realize you weren't paying attention and have to skip back several minutes. (Which can happen with a book too !)

Also, "podcasts" go quite a bit back : since it became practical to record radio (wire already, or did that only start with cassette tapes ?)

TV got that too with it's own tapes, but the portability and diversity was much worse until digital video got cheap enough.