The rating system used for movies and games are both voluntary self governing regulatory systems taken to preemptively regulate and censor the "community" without government censoring them.
Dave DuPlantis is right, that it cant be really rated due to a lot of things, but more aptly they refuse to rate it because it would discredit the stamp of approval and potently make an opening for government censorship of all games.
There's a few documentaries and articles about how the rating system works and point out the flaws in it, http://www.imdb.com/title/tt0493459/ is a look at Hollywood rating system. However the real reason it's there is not to provide a quality rating but to keep the government from censoring things and keeping the option for change in the "public", just don't trust the rating.
An example of not trusting the rating would be the Oblivion rating of M (originally T) when the game was censored without modification. The gaming community removed the censors and the game got a revised M rating. GTA has had the same problems though worse with the "hot coffee" mod that got the game rated AO forcing a reprint to lower it to M, and forced a rewriting of the ESRB's way to rate things.
See: http://pc.ign.com/articles/704/704726p1.html and http://arstechnica.com/old/content/2005/09/5311.ars