Why Don't They Label All Books as Easy?

So we are now into the second semester, and I have survived.  I have learned a lot and am ready to learn a lot more.  A new high school library aide began today, and he is getting lessons in how to shelve books.  Discovering the "E" (Easy) section stumped him.  His question: "Why don't they label all books as easy?  I would read a whole lot more!"