Actually, it's because Immanuel was born in the next chapter of Isaiah (8:8)! Verse 7:14 mis-translates the Hebrew word "almah" as virgin instead of "young woman". IF it meant to indicate virgin, a good Hebrew example of the word would be "betulah".
In addition to life choices and prophecy, the bible is also a history book. Isaiah 7 speaks of King Ahaz seeking comfort due to the alliance of Rezin (king of Aram) and Pekah (son of the king of Israel). The context is that by the age of (Immanuel's) maturity, the kings of the 2 enemy nations will be gone; in fact, killed (ref. 2 Kings, 15:29-30 & 2 Kings 16:9).
Further context is the Lord telling Ahaz that he could ask for a sign but Ahaz responds that he "will not try the Lord". Then comes Isaiah 7:14...but it would make little sense that the Lord's comfort for Ahaz would be the birth of Christ 700 years hence.
For other reference, check out Proverbs 30:18-20. Here, Solomon compares a man with an "almah" to 3 things: an eagle in the sky, a serpent on a rock, & a ship in the sea. The common bond? All leave no trace - after an eagle flies by, there's nothing in the sky to show it was there; a snake slithers away & leaves no trail; a ship moves across the sea & the water comes back together...after a man has been with an almah, there is no trace of the fornication between them. No...DO NOT go there because that's not what I'm implying! Simply: when we're addressing apologetics, it's critical to view it within the context of the circumstance at the time as well as have a working knowledge of language(s).
The Lord Bless and Keep You! (Numbers 6:24-26)...Michael