Perhaps the biggest difference between computer games and real-world games is that in a computer game, the player doesn't usually feel compelled to play a certain role.
Thanks to a variety of issues - saves, no respect for NPCs, no ability to improvise or express yourself - computer game players don't really like to stick to their role. Real world games use competitive goals, time pressures, and social pressure to get people to stick to their roles, and even that is often fairly shaky. Some games use these methods (most notably competitive goals), but that frequently makes them unsuitable for the general public.
I guess that's really all I have to say on the subject.