Because in-depth storylines sell games these days.
You know Square-Enix bread and butter Final Fantasy? Well, of course what everyone talked about in Final Fantasy X was the story!
Oh wait, no, everyone liked the graphics. The story was terrible.
I'm pretty sure that challenging gameplay and graphics are what sell games. If not, there would be the 'Tom Clancey: Press A' where you press A for each line of dialogue, and you unlock achievements by flipping pages.
People read books for story. People do anything else for excitement.