Frustrated, annoyed, or disappointed customers wont tell their friends to buy the game, which hurts the bottom line. Excited, giddy, and euphoric customers will buy copies for their friends and family as gifts, because they know it is the best present anybody could receive. Another issue is too many people cramming into the same area with not enough resources for everybody. Camping becomes commonplace. Monsters become scarce.
Players become bored.
Shopkeepers should become rich as they sell oodles of food and equipment to the hordes of adventurers. It begins to feel less like a virtual world in which role playing can occur, and more like a game with an unwritten ruleset involving racing to spawning monsters and negotiating with everybody around you for your turn at the next quest. Along those lines, these large-scale games are completely unrealistic. It is impossible for a player in that environment to be a real hero if everybody is saving humanity. There are more adventurers than there are citizens to protect.