Answer

First ask about the question itself. We actually were commanded regarding character refinement. Positive commandment 8 in Maimonides:
And the eighth commandment is that He commanded us to emulate Him, exalted be He, according to our ability, as it says, “And you shall walk in His ways” (Deuteronomy 28). This command was repeated, as it says, “to walk in all His ways” (Deuteronomy 10, 11). And in explanation of this it was said: Just as the Holy One, blessed be He, is called merciful, so you too should be merciful; just as the Holy One, blessed be He, is called gracious, so you too should be gracious; just as the Holy One, blessed be He, is called righteous, so you too should be righteous; just as the Holy One, blessed be He, is called pious, so you too should be pious. This is the language of the Sifrei (end of Eikev). And this command was repeated in another formulation, as it says, “You shall walk after the Lord your God” (Re’eh 13). And it was also explained (Sotah 14a) that its meaning is to emulate the good actions and noble traits by which God, exalted be He, is described, by way of metaphor, for He is exalted far above all.
Necessarily, the command is about our behavior: that we should try to emulate Him in the way He conducts Himself in the world. After all, He does not have character traits in the psychological sense that we do, so this command cannot include emulating the traits themselves, only the behavior. But the discussion here is about why we were not commanded regarding the refinement of the traits themselves, not the behavior. To that, Rabbi Chaim Vital answers that this is a precondition for our being commanded, and Rabbi Kook answers that here one who is not commanded and acts is preferable. But regarding murder and theft, according to Rabbi Chaim Vital’s answer it is obviously irrelevant. Yet even according to Rabbi Kook, one can say the same thing: after all, these are not prohibitions on traits but on actions—it is forbidden to steal and to murder. And here the principle is to do what is right, not specifically to act from the right motive. Therefore there is a command.