The OT is a book of eyewitnesses, but not to Jesus, but to God himself. God was actually talking to his people during this time.
Matthew is an Apostle of Jesus, so he was an eyewitness. Mark (John Mark) was not a witness, but he used Peter, who is an eyewitness to help him write the book. Luke was a companion of Paul, who was a eyewitness when Jesus revealed himself to him. John was an Apostle, so he was an eyewitness as well.