I mean, some of those personal interests were long term too. "Peasants need food within the next few days or they'll die": short term interests of the people. "I need aristocrats to take everyone's grain so I can make them be knights and keep people from overthrowing me": long term interests of the monarch. the people are very good at identifying their short-term interests (not starving,) but it's hard to figure out long-term collective interests (exactly how much grain to give up for however many armored guys on horses you need.) this is why modern democracies are usually republics.