##Because there is no reason to think he did. That we know this is not a reason to attribute to him - or to any other NT writer - the knowledge of later ages.
Maybe because that argument is good logic. If it’s invalid or otherwise wrong, please explain how
That second sentence has torpedoed your own argument, blown it up amidships, & sunk it without trace. A NT that does not yet exist, cannot be used to support doctrine - any doctrine. What’s the use of telling a Christian to read as Scripture a book that
- has not been written
- has not been recognised as inspired by God
- is rejected as a Gnostic forgery
In several churches, Revelation had the same status as the Urantia Book or the Book of Mormon has among Christians today. Others accepted it as Scripture.
“The 27 books existed in the local churches” =//= “all the books were at once accepted as Holy Scripture”; they all came to be accepted as Holy Scripture, in the local churches & in the Church Universal (though to this day there are some - the Ethiopian Church ? - who don’t include Revelation in the canon). Their final recognition is no proof at all that they were never recognised as anything but Holy Scripture: whether singly, as groups, or as an entire body. Revelation wasn’t. Hebrews wasn’t. Likewise others
Any honest but mistaken man, charlatan, lunatic or liar can claim his books are Divinely inspired

- Mohammed & Joseph Smith do so, but it is not exactly clear how they can be made to agree. Maybe a fight is the way to settle whom to believe:
grudge-match.com
Why should it be ? ## Let’s go through this:
- “Book X is written by an Apostle” does not show that Book X is inspired
- still less is it proof of Divine inspiration.
- And even that is no proof that it is to be kept permanently by a local church
- let alone by the entire Church
I have read them.
## “Scriptural” in what sense though ? Clarification is needed before this can be answered
## St. Paul, like Jesus, constantly “goes against Scripture”. Deuteronomy 21.23 says: “Cursed is he who hangs upon a tree”. St. Paul is untroubled - he makes the Biblical disproof of the validity of the claims made for Jesus into an argument for the truth of those very claims. See 1 Corinthians 1
If we reason only from the Tanakh-OT, Jesus was exactly the “deceiver” His enemies claimed. They found arguments against Him in the OT. In the year 30 or so, Scripture demolishes the claims made for Him. The Jews look in the OT - & they don’t see Him as Messiah. There is no reason they should.
As for “question[ing] what was written by an apostle of Christ” - I’m not: I am questioning dubious reasoning. And I’ve already explained that in his lifetime St. Paul was no more than “a mortal man”. And his own letters are evidence of this - not they are needed to show it. So when they believed what he said, his hearers were believing “a mere man”. To many of his hearers he was not a writer of Divine Scripture at all. He is to later generations, because his writings have had time to be received by the churches.
Some Judaising Christians claimed to be apostles - he didn’t accept that they were. To make claims is no proof at all of their truth. In his lifetime, he was no more than any priest or Pope or other Christian teacher is today - unless everything he ever said is in the Bible, which there is no to believe.