Re: How do Kings and Emperors become that in the first place!?
Major Chinese dynasties usually last 300 year ish give or take some.
Essentially, if we're only talking about the formation of Empires, then it is basically the usual logical step for humans. after all, humans are in reality pack animals. they survive because they work together, and to work together you need a head. to give you direction (much like how elephant herds work)
Notice how non-king/emperors systems only worked in city states before modern time? unless you have a population of generally well off people all living pretty closely together, it is pretty much impossible to use any other system before modern times. it is nearly impossible for a kingdom that encompass more than just a city state (or a city state + colonies, like Venice ) to pull off anything else. so notice how when the Greeks and Romans expanded, they also gradually turned towards monarchy?
So why is it usually heredary? The answer then is two fold, first is the natural instinct of man to keep the best for their offspring (which is again, a natural animal instinct) . but why does everyone else tend to go along with it? the answer is stability, because a non-orderly succesion typically leads to trouble, which could end up endangering everyone. So the succesion system would hopefully be as smooth, simple, quick as possible, and just keeping it a father to son thing is usually the easiest way to assure that.
The key to modern democracy / republic is the improvement of technology that increased both.
a. overall education level
b. spread of information (this is the most important really, since education is tied to this as well)
before this begin to happen, monarchy was simply the most logical way to managing any decent sized state.