Jesus isn't the only Biblical figure whose visage is unknown. Very little description is given about figures in the Bible. We don't know what Paul or the original twelve disciples looked like. In both the Old and the New Testaments, physical attributes seem to be mentioned only when they are germane to the rest of the narrative. For example, the glorified Jesus after he had ascended to heaven is described in detail in the book of Revelation. Mentioning physical attributes only when necessary to advance the message of the text just seems to be a convention of ancient Jewish literature.