Why Wasn't the Virgin Conception of Jesus Mentioned by Mark or Paul?

In this episode of the Cold-Case Christianity Broadcast, J. Warner Wallace continues his series on the virgin conception of Jesus. If the virgin conception of Jesus really happened, why didn’t Mark or Paul say anything about it? Does their silence mean the virgin conception was a late idea added to the story of Jesus? Is there any good reason to believe Mark or Paul knew about the virgin conception, and if so, why didn’t they mention it?