Was I really going to read a thousand page physics text full of scary math? While the race of librarians in the story is dying out, Borges does not want the same fate for thinkers around the world.
Short stories did not disappear, however.
They are a big part in the book because they create excitement and suspense for the reader, they are often times the reason for conflict, and they give the story different perspectives due to the variety of characters.