Matthew interprets Isaiah creatively, not in keeping with what Isaiah meant. The child’s birth is not miraculous in Isaiah, but the deliverance of Judah from a military coalition is.
Jesus isn’t true because he is miraculously predicted 700+ years before his birth. Rather, because Jesus is true, Israel’s story gets reframed around him.
If we think of salvation as a one-time deal, a transaction with God, the ways that the Bible speaks of salvation, save, savior, etc. (Old and New Testaments) won’t make very much sense—like this story of Zacchaeus.
“Hi everyone! The one we worship was crucified by the Romans. Come follow us.” This might not be the best way to start a religion in the ancient world.
Much is at stake theologically for Jesus not knowing everything. It means grappling with the implications of the incarnation, no matter how challenging those implications may be to our own theology.