The vast majority of consumers only pick up new products based on the little they know about it from a 30 second ad. I believe that is why the high number of sequel releases have been occuring recently. It's easy to sell a product that the people already know and trust. Controversy helps too. Game Developers have simply let it go that way, and over time have gotten lazy themselves.
When this tech was new, every game was too. Each release was some new twist to better suit the existing tech so it could try to live up to the potential of what it could do. The tech gets better? So do the games. BUT when the ONLY change is graphics...the gameplay begins to get stagnant.
Honestly, I think GTA3 unwittingly started the trend towards gameplay mediocraty. The people liked it so much that they wanted to be able to live out other fantasies using the same sandbox setup (Mercenaries, Dead Rising). RE4 came along, and now we have Silent Hill Homecoming, RE5 (duh), and Dead Space. And RE4 was just a smaller view of a smaller sandbox. First Person Shooters COULD be the thing that allows for some growth, but the only thing you can really do is SHOOT.
With the gameplay falling into each respective niche, so too go the stories involved, because how many ways can you explain why you're shooting people? Rockstar released Manhunt and there was no real story there, just a murder simulator. What else do gamers in this day and age do with our games? We kill things. Maybe, we'll solve puzzles, too, but that either is preceded or followed by the death of something at your hands. Sometimes that's the point of the puzzle.
Gamers are now asking "Why" a lot more. "Why am I supposed to be killing that guy again?"
"Because he's there." is losing it's potency as an answer. We were heros!!!! Link was a hero!!!
Think about how many games have come out now that have reduced you to just being a killer. The developers can do better.