Because in practice it often doesn't work out that way. Republics and representative government, especially in their modern conception from the 18th century, we

Because in practice it often doesn't work out that way. Republics and representative government, especially in their modern conception from the 18th century, were not simply a different way of appointing kings and their subsidiary advisors with broadly defined powers of "protection" over people under vassalage at their discretionary interpretation. Elections in most modern states don't confer that kind of supreme rule. It is the primacy of law, which constrains leaders from encroaching like that, and applied objectively to citizens in regards to their own rights and responsibilities that drives most countries you and I would probably say we'd like to live in. What you're describing isn't too tangibly different from the Spartan ephors.