As far as I can tell, it's because American game advertising studios want to be advertising Hollywood summer blockbusters. They are of the opinion that games come in three types: Action movie, space opera movie, and match-3... and nobody cares about match-3 games anyway, right?
I think a similar sort of thing happens in movies, if you look into it - although somewhat in reverse. Big action movies get action-focused movie posters and lots of advertising. Slower, cerebral movies - especially those without any gunfights to "justify" the thing - tend to have less advertisement and more esoteric posters - almost as if the director himself decided to have a crack at making the poster.
It's all about appealing to a target audience, and both American videogames and American movies think their target audience is, without fail, the meathead Superbowl-addict chips-and-beer demographic.