Because Paul received his authority and message by REVELATIONS FROM JESUS CHRIST, face to face and mouth to mouth. Just like Moses received his authority and message from God, face to face and mouth to mouth.
"..I have appeared unto thee for this purpose, to make thee a minister and a witness both of these things which thou hast seen, and of those things in the which I shall appear unto thee." (Acts 26:16).
"And it came to pass, that, when I was come again to Jerusalem, even while I prayed in the temple, I was in a trance, And SAW HIM SAYING UNTO ME..."(Acts22:17,18)
"It is not expedient for me doubtless to glory, I will come to VISIONS and REVELATIONS OF THE LORD."(2 Cor. 12:1).
"And lest I should be exalted above measure through THE ABUNDANCE OF THE REVELATIONS,.."(2 Cor. 12:7).
"For I have received OF THE LORD that which also I delivered unto you..."(1 Cor. 11:23).
"But I certify you, Brethren, that the Gospel which was preached of me is NOT AFTER MAN. "For I NEITHER RECEIVED IT OF MAN, NEITHER WAS I TAUGHT IT BUT BY THE REVELATION OF JESUS CHRIST."(Gal. 1:11,12).
If all the answers were contained in the Scriptures, why did not Paul do like a good sola apologist and give them a list of Bible verses?
St.Paul says " And no wonder, for Satan himself masquerades as an angel of light." (2 Corinthians 11:14)
What matters is not the "claim" of authority directly from God Himself, because that claim is extremely common in history, and often a disastrous source of errors.
What is important is living contact with Christ's chosen Apostles and their successors.
Paul says that much Christian teaching is to be found in the tradition which is handed down by word of mouth (2 Tim. 2:2). He instructs us to "stand firm and hold to the traditions which you were taught by us, either by word of mouth or by letter" (2 Thess. 2:15)
He says the household of God is built on the foundation of the apostles and prophets, with Christ Jesus himself as the chief cornerstone (Ephesians 2). He lays hands on other men to make them successors in his apostolic ministry (e.g. Timothy and Titus) and specifically acknowledges still others who, like Matthias, were also added on after the original Twelve:
"Greet Andronicus and Junias, my kinsmen who have been in prison with me. They are outstanding among the apostles, and they were in Christ before I was." (Romans 16:7)
What if any of these are in disagreement with each other? Then they have a council of the Apostles (as in Acts.) Century after century, human nature being what it is, there have always been disageements. That's why, century after century, in a manner that shows living continuity, there have always been councils of the Apostles' successors.
Please note that I am not making a detailed argument concerning who those successors are today. But if the church isn't just a failed attempt whose remnants are found old books and in the ashes of the past, if it didn't just peter-out after the first century, there must be an unbroken line of living successors.