Because it doesn't fix anything as I've explained. You're suggesting that people who have conclusively demonstrated not to care about incentives or punishments alike will all of a sudden stop being jerks because they have to make a post or comment. They won't, they will more than likely demonstrate yet once more that the punishment of making a post doesn't mean anything. You do run the risk of peaking their Joy by inviting others to downvote their posts/comments and in turn satisfying the jerks with the thought that others wasted their voting power on them. The best way to deal with jerks is to ignore them because without reaction to their nonsense they will move on as their satisfaction is hinged entirely on others reactionary responses.