Countries and the people who make them up also form alliances, hate betrayal, and hold grudges. If your country thinks it is acceptable to betray allies, you will soon have no allies. If you say one thing and do another and the gap is wide enough, eventually no other country will believe a word you say, even when you tell the truth.
Alliances between countries are not formed in a rational way just on a specific issue normally, but based on previous actions and previous issues, and old loyalties. To betray those loyalties just means no-one will feel any loyalty to you - that may work in the short-term if you are big enough but long-term it's a terrible strategy.
So in that sense countries are like people, though I agree in many senses they are not.
