the author's signature has value because the author has a very limited amount of time in his life to give out signatures. moreover not only it takes time to sign stuff but it also takes effort. these two combined, plus some subjective personal feelings about the author and their importance, give value to a physical signature. digital signatures on the other hand are effortless and automatic, the author doesn't have to do anything beyond generating a public key pair once

More like a torn out page with signature, without the book. The book itself is nearby, but another person holds it in his hands and can open it for you when requested.