Kings would be killed by their nobles if they did not defend their interests. A dictator does not just get to do whatever they want, they have to meet the needs of their political base which they absolutely have. Often that political base is a military or cohort of oligarchs.

Because you said dictators would be killed by the populace

Do you think “nobles, oligarchs, and military leaders” are the same thing as “the populace” or “the average person?” Can you read?