I don't have a solid block of 40 undisturbed minutes to listen to a talk. Give me a transcript and I can read a paragraph here and there as I do other things at my own pace. I might have ten minutes here, ten minutes there. I don't want to be constantly pausing/unpausing the video, or worse - switching between the video and my music.
Plus, if I concentrate, I could read a 40 minute talk in 20 minutes or less.
Basically, when I'm reading, I control the pace. I rarely watch videos that are longer than about 5 minutes (that aren't entertainment, which is entirely different).