Because surnames (i.e., last names) are generally a modern convention. During biblical times, to the extent surnames were used, they were generally identifiers of lineage. For example, Jesus of Nazareth might also have been known as “Jesus ben Joseph” (“Jesus, son of Joseph”), although he would *not *have been known during his earthly lifetime as “Jesus Christ” because “Christ” is not a surname but a title that means “the anointed one” (i.e., Messiah).
