More accurately, the question is why the unknown author of the book of Mark, who never claimed to have first or even second hand accounts of Jesus, used that me

More accurately, the question is why the unknown author of the book of Mark, who never claimed to have first or even second hand accounts of Jesus, used that metaphor in his story. The subsequent two usages by the anonymous authors of books Luke or Matthew clearly lifted that story from the earlier work - I don't think that's in dispute here.