It is probably the same reason as why books (text) are still the main medium. Movies may be more popular entertainment, and comics might be preferred by some demographics, but everybody write something, and everybody read something, being it a facebook post, a tweet, a novel or a 10-tome encyclopaedia.
Nobody says that books kinda suck and should be replaced by pictographs on papyri, but somehow many programmers at least have a phase when they think that having to program by drawing shapes could somehow be easier than writing textual code...
Geometry books don't use text as the only form of communication. Neither do books about architecture, mechanical engineering, anatomy, circuit design, etc. Imagine a world where you used only text to convey all these ideas and their relationships. Computer programming today a bit like that. Why should we assume that pure text is the best and only suitable form for representing automatic computation?
Nobody says that books kinda suck and should be replaced by pictographs on papyri, but somehow many programmers at least have a phase when they think that having to program by drawing shapes could somehow be easier than writing textual code...