@OP: I agree on the violence part, it does interest kids since they think it's something that makes you mature.
But there's also another point of view, and this is about shooters. I actually wrote something similar to another topic. So, brains are in a state of constant change in ages between 12-25 (could start before 12, but should end at latest in the age of 25) and the brain chemistry is somewhat disturbed until brains reach it's maximum. These brains also need a lot of testosterone, which you can give to them by playing a simple fast-paced game, which really don't require thinking, only reflexes and preferrably constantly. When they feel like they've accoplished something (in this case for example fragged someone online), brain rewards itself with a hefty amount of endorphines, which is comparable to injecting morphine (endorphine actually is relative to morphine). When you're around 15-16, the change is in it's maximum and at that age, the games i mentioned, hit the best.
The reason why they play this kind of games, is that it's competition, if you don't compete in games, you compete in something else, sports, driving etc. It's all about natural behavior of becoming "hunter", "warrior" or even "fisherman", in other words, the alpha-male, who gets to choose first with who to reproduce, get your own genes forwarded before others.
Now, after you reach your personal alpha-male status, you also calm down, compete only that much, what's necessary to remain your status.
The reason why gaming in general is seen "kiddy" is the competition aspect in games, which is considered for immature unsecure people, who still haven't found their social status. When non-competing, in other words, fun, is considered to be more mature, since it's something that secure matures do to pass time. So, basically you compete only with people, who can challenge your status, or as a challenger, you compete people with higher status.
@dgenerate: If half of the money EA makes with Wii goes for developement for other consoles, it means certain financial irrationalities. Which means, that they will focus on Wii, since it makes more profit. It's as simple as that.
@gebx: Maybe the people grew out of Halo? Anyway, it doesn't mean, that everyone who bought earlier Halos are Halo fans. The newest Halo was bought by people who are fans, people who just saw it as quality game for 360 and people who haven't played Halo ever and wanted to find out what the hype was all about.
@Kyros: Innovative gameplay, game depth and sophistication have hardly anything to do with hardware it's running. But at the moment 360 and PS3 games have more depth and sophistication than Wii games, on average. But things can change quickly. The best (independently) selling game on Wii happens to be the most sophisticated and has the most depht of Wii games. This should tell the developers, that this kind of games sell on Wii.