Altogether now: "Yes, we're all individuals!"
Interesting question. As a random bloke on some forum - and with no qualifications at all - I'd say it's a combination of our conditioning and our need for reassurance by our peers. Conditioning because there isn't enough time for us to experiment on which berries are poisonous and which are safe to eat, whether we should vote Labour or Conservative. We look to people whom we trust, be it parents, friends, role models on TV, and allow them to influence or dictate, depending on how strongly we trust them. And reassurance because getting that +1 or that "like" affirms that we've won somebody's approval, a process we're then incentivised to repeat.
Sadly neither of those things are compatible with being overly analytical since that involves a departure from the herd, with the size of the departure being tied to society's tolerance for those that question. Every day I work with people who are adverse to open debate and other people who relish it. (I'm resisting the urge to introduce geo-political stereotypes here, but if you've worked with Americans and Scandinavians on the same team you know what I mean)
Suffice to say that it can make you feel isolated when you have arrived at a conclusion based on logical analysis and find yourself surrounded by a majority who have not only arrived at a different conclusion, but can't articulate their thought process and just get abusive instead. No wonder it's easier to just follow the herd.
Feckin' nightmare! You should meet my family ...