People dislike having penalties. So the only other alternative is rewarding the people who aren't supposed to be penalized/ aren't supposed to be "the problem".
If you don't want being good to give rewards, then you'll need evil to be penalized. If you're talking about real life in your example, it's true that the main reward for being good is a social one (emotional), but by being evil you gain rewards more towards the materialistic side of things at the cost of your freedom. Would you want to lose your freedom in game? This goes hand in hand with saying, "would you want being good to really serve no real game purpose other than the RL one, which you can go and get without any problem?" You can't be evil in real life without any problems, that's why people do it in game...