Truth is higher than justice. The affairs of state would be very disturbing to a scholar.
War interferes with research.
Thus, for Aristotle, the happiest life, the greatest human good, is the fulfillment of the philosopher. The good ruler, himself denied the leisure, peace, and joy of scientific knowledge, orders the city so that the intellectual scientists may be fulfilled. But this final end is most adequately realized not in the ruler, but in the scientist. Leisure, not work, is the basis of culture. And the ruler has very little leisure.