It's not as if the New Testament writers came along and said, "The culmination of Old Testament books is more books, New Testament books." In some ways they thought instead of the culmination of Old Testament books being Christ himself, the word incarnate as the opening verses of Hebrews 1 put it. In the past God spoke to the fathers by the prophets, but in these last days he has spoken to us by his son and the son is revelation.