Paul was not simply teaching the same things by word of mouth that he wrote about in his letters. Read the Didache or other historical documents of the time. The rules around baptism, celebration of the Mass, the Church hierarchy were all discussed in detail in the first century. These details are discussed in Scripture only briefly but were very well developed outside of scripture.
Remember, Paul often focused on what was going wrong in the Christian communities he was writing. His letters did not serve as a manual for behavior - that was known by tradition and by things like the Didache.
Jesus had foretold the weeds springing up and Paul affirms this was occurring.
What sorts of behavior did Pauls letters overlook?