Because the laws are about particular things in the real world that have nothing to do with the legal system. They are frequently about scientific matters, for example. What constitutes a threat to public health? What constitutes pollution of a waterway?

When Congress authorizes an agency to maintain, say, clean drinking water, it entrusts scientific experts to determine, based on the most up-to-date evidence, what constitutes a pollutant that is harmful to human health. We do not need Congress to pass a new law every time we get new scientific evidence that a particular chemical (say, PFAS), is harmful.