Audio/video is also very inconvenient when you need to slow down your processing rate. You have to tweak playback controls for this, while dealing with pure text only requires a slight adjustment of your eyeball muscles. And similarly when you encounter parts that can be skipped forward easily: text just requires some more motion of your eyeballs, while recorded audio/video is a lot more awkward to deal with.

Nota bene: A great lecture is worth a lot. But a great text that delivers the same message is worth a lot more.