The straightforward answer is that it is up to God, not us, when he chooses to leave us inspired books. The books of the Bible were given to achieve his purposes. There have been other times in history when God did not choose to leave his written word; actually nearly all the time this has been the case. Perhaps a more fruitful question is why God chose to leave us his written message at all. Clearly the answer is that God calls us to repentance and to the hope of resurrection and of his kingdom. If this can be achieved with what he inspired to be written, then who are we to demand more?