Why would it take a book, Paul and the author of Hebrews did so within a relatively short amount of effort?
Similar to Abraham's request to God, that if He found at least 10 righteous men in Sodom, would He spare the entire city, God said yes. God, due to the righteous efforts of 1 man, agreed to exonerate all men on his behalf, provided that they accept his Lordship and Messiahship. Christ as a man, met this requirement by fulfilling the demands of the Law, allowing God to abrogate the Law which removed the condemnation that came with it, and thus, absolve man of guilt.
But, fulfilling the Law also means loving God with all one's heart, and one's neighbour as themselves, not just the other 613 decrees.
Thus, the Saviour or mediator can never be God Himself, in order for this judiciary requirement to have any efficacy, or even make any sense.
You are confusing two principles, or expressions rather. The foolishness of God, is not actual foolishness to the wise. It was a paradox that Paul was stating. Nothing that God does is foolish, it only may appear so to the unenlightened and depraved.
This is why, as a Biblical student, we determine that there is no room for veritable nonsense in God's Word. So when a doctrine reaches the level of incomprehension and absurdity, in all its facets of theology (Soteriology, Ontology, Bibliology, ...), we categorically denounce it as heresy.