Why don't you find this language about the old Adam and the whole human race in the Gospels?
This language is absent from the Gospels because Jesus was dealing primarily with Jews under the Law, not yet opening up the whole human problem. Paul, by Holy Spirit inspiration, later breaks out beyond Israel to address the entire human race and its old Adam.
Les explains that this language about the old Adam and the whole human race is not found in the Gospels because Jesus did not get into that part of the whole problem. Jesus was more or less dealing with Jews under the law. But Paul, by Holy Spirit inspiration, breaks out and delves into the whole human race, not just Israel or Jews. The law gives the knowledge of sin, that is, the knowledge of our old Adam. Salvation is not of works, lest anyone should boast; it is only the work of a merciful God.