You know today's controllers : you sit somewhere and play.
The wii's controller makes you move, you'll sometimes end up exhausted. Beating an opponent does not mean you simply have better fingers, it means much more; you play with your entire body, not just with two hands. You'll have to do big moves in order to catch a ball, just like in reality. Playing with a classic controller has always been a bit sad because it has never tired anybody (not considering >4hours LANs

).
In fact, it gives you the fun of the reality and the occasion to have a minigun or a rocket launcher. Moreover, there is a social thing : it seems meant to be multiplayer.
Maybe that the best example is parents who tell their children not to stay in front of the television as a couch potato while they can do sports outside.
Maybe that
wikipedia will say it better:
Quote
Another widely publicized effect of endorphin production is the so-called "runner's high", which is said to occur when strenuous exercise takes a person over a threshold that activates endorphin production. Endorphins are released during long, continuous workouts, when the level of intensity is between moderate and high, and breathing is difficult. This also corresponds with the time that muscles use up their stored glycogen and begin functioning with only oxygen. Workouts that are most likely to produce endorphins include running, swimming, cross-country skiing, long distance rowing, bicycling, aerobics, or playing a sport such as basketball, soccer, or football. However, some scientists question the mechanisms at work, their research possibly demonstrating the high comes from completing a challenge rather than as a result of exertion. (Klosterman) (Altman) There is some recent evidence that endogenous cannabinoids are responsible for "runner's high", rather than endorphins. (Endocannabinoids and exercise, by A Dietrich and W F McDaniel, May 4, 2004 bjsportsmed.com). Studies in the early 1980's cast doubt on the relationship between endorphins and the runner's high. There were a couple of reasons for this doubt. The first was that when an antagonist (pharmacological agent that blocks the action for the substance under study) was infused (eg naloxone) or ingested (naltrexone) the same changes in mood state occurred that happened when the person exercised with no blocker. A second piece of evidence is much more simple. It turns out that scientists cannot make a runner's high occur in the lab with any certainty. This makes it very difficult to study much less prove that endorphins cause the runners high.