Text on a page allowed the reader to rapidly scan back to previous points to make connections between concepts. Similarly you can scan forwards quickly and then return and read more closely after identifying key words and topics.
Text gives additional structure to the information with clearly delineated paragraphs, sections, chapters and italicized or bolded words and phrases. Citations and footnotes, asides and figures can be referred to more easily than in a video or audio where they must occur linearly.
Text is also more easily modified and marked up by the reader making referring back to important parts simple, being able to do so at a glance of the page rather than scrobbling through the video or audio.
I’m sure others can think of many more but the point is that text is more information dense and less linear than video/audio.