The stereotypical powergamer runs around at top speed killing the most efficient monster in terms of experience or money gained over time, often stopping only to share a cordial, Sup d00d. Unfortunately, no game has ever rewarded the roleplayer, and so these perpetually first level players have yet to find a comfortable place in online gaming. 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.