Why would the Apostles and those that traveled with them write the Scriptures then?
Just the Oral Law was not written until the Christian Era...i.e. what we call today the Talmud. The Old Testament Scriptures of course had been written all along.
You do raise an interesting point though. Peter wrote no Gospel. James wrote no Gospel. Andrew wrote no Gospel. It seems they felt not much of a pressing need to write down what they had learned from the Master, if they entrusted it to very minor Apostles like Matthew and mere disciples like Mark and Luke.