(Some) Players go to RMT’ers because they sell for less than what CCP does. Whether it’s plex or isk, the only way CCP could stop the RMT’ers by going this route is to sell the plex/isk so cheaply that the RMT’ers would be better off gold farming in another game. This, however, would drastically hurt CCP’s financials and have serious ramifications for the in-game economy.
Also, as Ms Steak said, selling isk directly to players creates an isk faucet. Selling plex does not.
Finally, for anyone that might be thinking of buy stuff from an RMT’er, you stand a good chance of being caught, you ill-gotten gains will be seized, you will not be given a refund by the RMT’er, and you put yourself at increased risk for having your account hacked. And, if all of that wasn’t bad enough, you are hurting the game by doing this.