Interesting thoughts on why game worlds outperform virtual worlds like Second Life:
In this telling, games are not just special, they are special in a way that relieves designers of the pursuit of maximal realism. There is still a premium on good design and playability, but the magic circle, acceptance of arbitrary difficulties, and goal-directed visual filtering give designers ways to contextualize or bury at least some platform limitations. These are not options available to designers of non-game environments; asking users to accept such worlds as even passable simulacra subjects those environments to withering scrutiny.