That's why game CEO's are game CEO's, not smartphone CEO's.
Although Apple probably has the next iPhone's details pretty much finalised, they'd be subject to last minute changes. Everyone would get hyped up about a feature, and Apple might end up having to remove that feature due to last minute issues.
The iPhone 4S may well have been plan B. If Apple had released iPhone 5 promos, imagine the chaos and damaged reputation if they had to announce that it was delayed till next year and instead give us an improved iPhone 4.
The game industry is nothing like the smartphone industry. People tend to buy games not long after they are released. It isn't like game sales are still going well months and months after the game as been released.
Also, just because there's a new game, doesn't mean the old game is redundant. People will still buy the old game, especially if it is part of a series (e.g. Final Fantasy).
Games are not a huge commitment in the way that a smartphone purchase is.