When the founders of this country were debating the principles by which we would be governed, the first premise on which they agreed was that there would be no king. They knew what it was like to live under a tyrannical monarch. They also knew from study and analysis of the ages, how Ruler's Law did not normally work out well for people in the "realm." That's the problem; the people are the property of the king and therefore have no say in their lives. They do not have inalienable rights given them from God, they only have the rights issued by the king, and what the king gives--the king can take away.
Ah, but what if the king is wise, kind, and generous? What if the monarch treats his subjects with fairness and concern? Have you ever, in studying the annuals of history, run across such a benevolent dictator?
Isn't there a saying that "power corrupts, and absolute power corrupts absolutely?" Something about the basic human psyche makes it nearly impossible for one person to possess huge amounts of money without greed, or be given unlimited power without despotism.
The Founders did not want a king, nor did they want a government behaving like a monarch.