There is a slight misunderstanding here. The Justinianic Code was a collection of previous laws, one aimed at weeding out contradictions and repetitions, rather than a wholesale invention of new laws. The Romans regularly created new laws and the Twelve Tables were never the one and only 'source' of legal regulations. One of Justinian's own laws noted that his committee of lawyers had to deal with 1,400 years of legal confusion - each emperor had different policies and thus issued laws based on their circumstances. A law from the time of Constantius II banning officials from making religious sacrifices would for example be difficult to fit together with older laws dealing with pagan piety. Justinian's team removed these anachronisms to make litigation smoother and to remove various legal loopholes.

Additionally, there were already two unofficial collections of legislation from the fourth century, and then there was the more famous Theodosian Code from the fifth century. Justinian and his successors would also continue to issue novels (new laws), so the Roman system continued to evolve as well. Like any legal system, it has to change in order to stay relevant. Justinian's reforms built on past precedents and his grand project, which also extended to the collation of non-imperial legal works and the reform of the legal education system, was quite successful in that regard. The emperor was still perceived as someone bound by law (even though in reality they can generally overstep its bounds), so the issuing of new laws and the clarifying of old laws had always been a key part of what it meant to be a Roman emperor. The changes Justinian made were therefore entirely conventional, one made more impressive because of the sheer scale of the project and the speed at which it was completed.