Firstly, we should understand that every word found its way into the dictionary when it is popularised by some great poets or writers, or sometimes by the people i.e. when the word is being used by common people and gets acceptance. 

• The word zzz found its way into the dictionary because it was widely used by people around the year 2006 - 2008 to convey the idea that someone is sleeping in slumber. 

• So it is one of those words that get added to the dictionary of English because it was popularised by people.