There has been over a decade of propaganda telling people not to trust each other... all post-9/11 messaging intended to create insecurity in the population.
Yes, but that propaganda was nothing compared to the much more advanced and sophisticated level of indoctrination techniques we have in more modern history via mainstream television and radio.
And, don't forget WWII had wonderful internment camps and plenty of racism towards blacks in America. So it wasn't the most trusting place for everyone.
All across the Internet, people like you and me would post about how incompetent the average American is. We would aspire to be independent, logical thinkers, but unfortunately, this mindset almost requires the distrust of other people.
Now that the Internet has become widely adopted in the mainstream, this type of thinking pervades, and perhaps has penetrated the mainstream attitude.