Its not that games generally have poor storylines its just that people have unrealistic expectations about what makes a good story. The fundamental purpose of a story is first to entertain. If a story fails to entertain then any merit it may have in terms of education, morality, intrigue or any other positive value you feel a good story must have is irrelevant as noone would care to engage with a story which fails in its fundamental purpose.
In storytelling, often the simpler a story can be made, the better. It has always been a coveted ability of the better artists to convey more with less. A great writer can sometimes describe in a sentence what an average writer might meander for a chapter to explain. Just because a story is simple in structure doesn't mean that great meaning or great artistry cannot be portrayed within. Often by boiling away the parts of the story which aren't requred the writer can focus on the areas of greatest import to the story. Many of the stories which have been lauded as great have relatively simple plots which outline their overall structure. This is not a hindrence but a strength that intellectuals often overlook in great stories.
Games are a medium unto themselves and should be judged as such. They are not movies and they aren't books. Whilst they share many of the characteristics of both they shouldn't be judged by the standards of either. Games are defined by interaction and any stories told are generally defined by such as well whereas books and movies are a passive and non-interactive medium. In almost all cases the player is a participant in the games story, even going as far as creating the story for himself with some games.
Tease.
