Consider a single human neuron. An axon and up to approximately 10,000 dendrites that can attach to other neurons. Not very intelligent in and of itself. Some neurons are somewhat more effective than others, but all in all, they're pretty much the same.
Now consider a human brain. It has on the order of 10,000,000,000 neurons. And approximately 100,000,000,000,000 connections. A human brain is intelligent (at least most of them are). 10 billion neurons with 10,000 potential connections each can exist in one of approximately 1 x 10100000 possible connection states (that's a one with one hundred thousand zeros after it). So the brain is far, far, far, ..., far more than the sum of its parts and that's why intelligence emerges from it.
Consider a single human. A brain and the ability to maintain connections with up to approximately 1,000 other humans. Some are fairly intelligent in and of themselves. Some humans are smarter than others, but all in all, they're pretty much the same.
Now consider a human society, for example, the United States. It has on the order of 300,000,000 humans and approximately 300,000,000,000 connections between individuals. 300 million individuals with 1,000 potential connections each can exist in one of approximately 1 x 1017000 possible connection states (that's a one with 17,000 zeros after it). So the society is far, far, far, ..., far more than the sum of its parts.
But more importantly, any one individual can't possibly begin to have the intelligence, knowledge, and experience compared to society as a whole. Even a group of very smart individuals can't begin to compare. The gulf between them is so many orders of magnitude that it boggles the mind.
Granted, the metaphor isn't perfect. The neuron connects with all 10,000 other neurons at the same time. People are more limited in their communication capabilities (though communication technology is changing this). Nonetheless, because of the exponential nature, large groups have far more capacity for information, knowledge, and analysis, than small groups or individuals no matter how smart the individuals are. That's why democracies (or representational varieties thereof) and free market systems work so much better than centrally planned and/or dictatorial/totalitarian systems.