Because it's an expression in french. Some expressions may contain no article or minimum words. Or for the memorization, or just because it's an expression FYI: http://www.expressio.fr/expressions/pierre-qui-roule-n-amasse-pas-mousse.php Some other expressions: http://www.linternaute.com/proverbe/recherche/m/4/

Often expressions are coined by masters of the language. They are allowed to breach the rules. Like Shakespeare making up words and people coping him afterwards. Sometimes expressions are just old. Words have been dropped overtime because they are no longer needed to understand the sentence (since it's now well known).