I think there's another reason. Puzo, like Ian Fleming of James Bond fame, could never resist passing onto readers every bit of "erudition" he picked up along the way. He spent plenty of time in Hollywood, so he passed on Hollywood stuff to his readers. Somewhere, he learned about surgery, hence the technical stuff about Johnny's vocal chord warts and Lucy Mancini's operation. His absorbtion with Hollywood is one of the main factors that makes "The Last Don" so insufferable. In "The Godfather," it's at least tolerable because the Don is interested in expanding into Hollywood and exerts his influence several times on behalf of Johnny Fontane.