Socrates himself says a couple of times that he hesitates to make the claim, knowing how odd it will sound. And in the part of the quotation I elided above, he notes that existing philosophers, assuming there are any, will probably have to be forced to rule. This press-ganging of the wisdom-loving soul parallels a more familiar argument, namely that anyone actively seeking political power is thereby disqualified for it precisely on that account. Only the person who does not crave control can be trusted to exert it.