Developers are still figuring out how to tell stories in an interactive medium, the industry is still relatively young.
Also, a game story doesn't have to be particularly good as the interactive element is what draws the player in and immerses them in the world. Even some games that are applauded for having amazing stories actually have simple or mediocre storylines that are told very well. I think Mass Effect 2 would be a prime example. Simple over arching story told brilliantly through the individual character motivations.
In many ways, games have yet to find storytelling methods that don't heavily borrow from film or books. Storytelling is usually done through cut-scenes or through text. Some games use the interactive medium more uniquely like Half-Life 2. The story is told completely in first-person and you are always in control. It's up to you to observe the details of the story through the world. And when they do employ cut-scenes, it's because Gordon Freeman (you) have physically lost control (e.g. your trapped, or something is physically holding you in game).
I don't think it's the story that matters so much as the telling of the story.
