Why do good employees leave or quit?

'Good employees' is too generic and needs to be specified in such a way that it describes their output / productivity in accordance to their posts or roles.
To start the discussion it would be better if we say 'productive employees' or 'high output employees'.
If so, most likely because they are under estimated or poorly judged or traeted.

No good enviroment

Ramy! How you doing ? Can you be more specific?

1.
Management demands that one person do the jobs of two or more people, resulting in longer days and weekend work.
2.
Management cuts back on administrative help, forcing professional workers to use their time copying, stapling, collating, filing and other clerical duties.
Management puts a freeze on raises and promotions  Management doesn't allow the rank and file to make decisions or allow them pride of ownership 5.
Management constantly reorganizes, shuffles people around, and changes direction constantly.
6.
Management doesn't have or take the time to clarify goals and decisions.
Therefore, it rejects work after it was completed, damaging the morale and esteem of those who prepared it.
7.
Management shows favoritism and gives some workers better offices, trips to conferences, etc.
 

actually im going to brief a/m answers :-
1- job doesnt fill his ambitions ( can be salary ,,,, products ,,,company scheme ..motivations ,,etc )
2- Inviroment and atmosphere ( some people prefer calm atmosphere and cooperative team ,,some managers cant provide such atmosphere so ,,either u wait till he goes or find first best other job shown to u ) In general ,,,shouldnt be an obstacle as u work hard and been granted good relatively to others in same company ,,,,
3- change to better >>>sometimes u have all u need ..nice company ..nice team ...nice salary ..
what else u need more >>> BUT ...change to better income in interval of 5-10 years would ensure new experience >>
4- if u r under estimated or bad treated >> jump in next new job ....they will call you back ..if they are good in their field >>>>>>.believe me Good luck everyone ,,,best rgds