They were written by that time. That's all. Canonization did not take place until the end of the 4th century because many apologists and churches used additional books.
There is no doubt that most churches used pretty much the New Testament as we know it today, but only in addition to many other brooks now no longer considered acceptable. And this is the crux of the matter.
The Holy Spirit led the first generation of Christians following the end of the Apostolic Era to compile the Canon.
This is such clero-romnticism! Then who told them 200 years later to get rid of the books they were using all along supposedly under the Holy Spirit's guidance, such as the Shepherd of Hermas, the Didache, the Book of Enoch, the Epistle of Barnabas, the various Apocalypses, and various Gospels, etc.?
Maybe the inability to see the work of the Holy Spirit and to insist that the Canon was put together hundreds of years after the fact is at the root of your doubts about Christianity. I suppose I would look at it the same way if I thought a bunch of guys put everything together.