I tend to cheat when a game annoys me or bores me. Though "cheating" is a big word. I prefer to say that I'm altering my single player experience to gain more enjoyment out of the product. It's the same reason people mod games, they like their game... but they don't like certain aspects of it so are gonna go around this.
Multiplayer is, indeed, not the same. That's a manner of playing fair and good ethics. Single player is just you vs the computer, and sometimes the computer cheats first.
Personally, I have an issue with a lot of current gen trends. Flagging people who alter their single player experience is encouraged when it's directly modding the game (which is technically against the law and, depending on the game and it's developer, can lead to trouble) but people who use in game provided cheats get flagged. There is an influx of games that are discouraging the sale/trade of used games. There is an influx of "online or nothing" kind of game content (or the ability to play the game, period) which stops folks who may not get online.
The current trends in gaming, period, especially on the business side of things, really reads like the music and other industries. It's clearly about doing everything they can to control the way you use your purchased content and then shame you publicly for doing it anything other than their way
Gameshark, and the like, don't exist because they directly alter and mess with game code. Or, at least, that'd be my main guess. That and newer systems have fail-safes that prevent that kind of thing. A lot of times you need to crack your system and a commercial product that encourages people to jail break their system ain't gonna go over well with everyone else. (Especially with this influx of attempted anti-piracy.) I'd say it's more to do with difficulty to make those things work and the online integration than anything else. The only recent Game Shark devices I could even find were for the DS Lite and everything below that.
However you can still get Action Replay, and the like, to allow you to get game saves. Rather than cheat all the costumes, you can nick a full save of Marvel Ultimate Alliance 2 which would have everything unlocked already. Then you can play normally.
Plus it is cheating, and given a choice between allowing people to cheat or enforcing the moral normative they're going to come down on the moral normative. :\
That's always good. Rich, even! Lets force our views of ethics and morals on others... especially when they don't actually apply to real life.
Little FYI, cheating is actually encouraged in reality. In fact, cheaters typically win because they are willing to do anything to win.