You ever think about the word "textbook"? Like, yes, books usually contain text. Why did _that_ become the word for "the book you use in your formal studies"? "Text" book as opposed to what other kind of book?

Apparently in the early 1700's, it referred to a book with wide spaces between the lines to take notes.

Late 14th century, "text" came to mean "an authoritative writing". So it grew out of that, but it's a strange word to pick for "authoritative" when all writing is text.