Quite correct, however in addition to the above, the fact is that paper disintegrates after a certain amount of time. Almost all our ancient texts are copies made by monks in Carolingian times (9th century). To find something older than that is due to tremendous luck. Yes, in very dry climates such as the sands of Egypt we do find some older texts, but by and large all paper from that time has dissappeared. There certainly was no reason to copy such papers as those ordering a census hundreds of years later. Censuses were quite common in Roman times for the purpose of taxation so there is no reason to doubt that there was such a census at the time of Christ's birth.
And there are still extant copies of the pieces of the New Testament dating to the 2nd Century. The NT by far the most reliable record of antiquity.