Why do Christians still believe in the virgin birth?

The Christian idea of a virgin birth is derived from the verse in Isaiah 7:14 describing an "alma" as giving birth. The word "alma" has always meant a young woman, but Christian theologians came centuries later and translated it as "virgin." This accords Jesus' birth with the first century pagan idea of mortals being impregnated by gods. Almost all Bible scholars know that this is a mistranslation, and there is even footnotes on some bibles correcting this mistranslation.