I believe we have genetic social-survival strategies. Richard Dawkins wrote about Evolutionary Stable States (ESS) where some percentage of the population being a certain way becomes stable, and if that percentage tips, that creates the pressure driving the percentage back.
I believe there is a large percentage of people born with the 'Follow the largest pack' strategy, and no matter how irrational that pack becomes, that is their survival strategy and deep endemic fear prevents them from leaving the pack. They are sheep.
You and I were not born with that strategy.
Similarly if the population of free thinkers gets too low, or too high, some kind of pressure brings it back to the stable level. I'm not sure everyone being a free thinker would work out well. But without us, the pack has no corrective force.