I'm not sure how it became the norm in text adventure games, but for platformers part of it was a hold over from games being designed to be quarter eaters in arcades. Kids that played those became adults that made the next ones. I /suspect/ that a similar thing has happened with the kids that grew up on the N64 and gamecube had four player local coop as the ceiling and now lots of games that probably could have more than 4 players at a time (albeit with balance probably thrown out) are capped at 4. Looking at you, concerned ape :|