Ah, because they only use on verse and ignore the others. Again, if Jesus "fulfilled" this sign, then when he got old enough to know right from wrong which two kings that were threatening to attack Judah and King Ahaz died? The two kings died back in Isaiah's time. Plus, the problem with the Hebrew word being translated as "virgin". Isn't it important to check the original language sometimes and make sure it was translated correctly?
Matthew 1:18 This is how the birth of Jesus the Messiah came about: His mother Mary was pledged to be married to Joseph, but before they came together, she was found to be pregnant through the Holy Spirit. 19 Because Joseph her husband was faithful to the law, and yet did not want to expose her to public disgrace, he had in mind to divorce her quietly.
20 But after he had considered this, an angel of the Lord appeared to him in a dream and said, “Joseph son of David, do not be afraid to take Mary home as your wife, because what is conceived in her is from the Holy Spirit. 21 She will give birth to a son, and you are to give him the name Jesus, because he will save his people from their sins.”
22 All this took place to fulfill what the Lord had said through the prophet: 23 “The virgin will conceive and give birth to a son, and they will call him Immanuel” (which means “God with us”).
Luke has a birth story, but he doesn't quote Isaiah. The two birth stories are very different. And that's it. No more talk of a virgin birth in the gospels and only the one verse quoted by Matthew.