The birth of Jesus is also only mentioned at all in Matthew and Luke. Following the logic, since only half of the Gospels mention that means we should discard the theory that he was ever born at all.
In 100% of the cases where a birth narrative is present, the miracle of God becoming man is mentioned. Indeed, in John, even where the birth narrative isn't present, the incarnation - that God became man - is explicit.
Neither John nor Mark say anything about the birth of Jesus at all, so it is not surprising at all that there is nothing about the virgin birth.