Jeff Spock @ Gamasutra: "Part of it, I think, is inherent in the media. The player-character tends to be relatively generic; often either a voiceless cypher (Gordon Freeman, Master Chief) or a relatively unimaginative remodeling of the wise/tough guy that we have known and loved from Bogart's Rick to Ford's Indiana. The player needs to identify strongly with the main characters, want what they want, and love them enough to endure tens of hours of their trials and tribulations.
Too much "character" in the main character can turn off players; not everyone wants to walk in the shoes of a metrosexual angst-ridden teenager with a gravitationally impossible hairdo (yes, I am talking about Japanese RPG's).
So I think that both writers and designers play it safe with the main character. Easy to like, based on well-known and well-loved stereotypes, a comfortable pair of shoes to put on."