Because it CAN'T BE DONE ON IPB, the software this forum is run on. It would need all members to be part of some moderation group, which won't happen. (Hours and hours of work just transferring members to different groups, not to mention there's no way to make new members part of a moderation group already, so you would have to add them there manually after they make their account.)
Just report the OP, and ask for it to be closed. The amount of work to implement this idea isn't worth it.