IMO, multiboxers enjoy being different from the norm. If developers created a game with multiboxing in mind, then it would take that additional step of encouraging the playstyle thereby ruining the niche. A good amount of the fun in MB comes from trying to figure out how to do things through macros, strategies, etc that other players need multiple users. If 50% of the WoW players were MB, then it would likely lose its appeal to many current MBs.