Answer

A good question, and one that has already been answered here more than once.
But before I answer, let me ask you: do you think the laws of ritual impurity and purity were instituted for some moral purpose? And what about forbidden foods? And sacrifices? And holidays? I assume that at least regarding some of these you would agree that they were not. Do you understand the purpose of all of these? Do you know how to derive the details of the laws from that purpose? I assume you would agree not. So how, then, do the sages interpret these Jewish laws anyway? That question arises regardless of my own views.
And my answer is that interpretation can be done even without knowing the purposes. On the contrary, in Jewish law we do not require the reason of the verse. And nevertheless, no one disagrees that the sages must and can interpret. Interpretation can rely on the language of the verses, as well as on the spiritual intuitions we have.