Wait, I must be missing something here. If people are nice to each other and that's a bad strategy, surely you get to wipe the floor win them (i.e. fulfill your objective of winning) and if people are nice to each other in a way that makes them hard for you to beat, surely it is you who are playing the worse strategy by not doing the same?

In my group of friends, whether it is Risk or Monopoly, being nice makes it much easier to win. People are happier to enter into mutually beneficial agreements with nice people who they know are honest and keep their word.