Humans are social creatures. We're biologically programmed to try and fit in with the norm, plus the conditioning from our environment also reinforces this behaviour.
As a result, most of us don't think for ourselves. Or if we do it's usually only partially. We'll be open towards some things, and close-minded towards others.
Maybe this made sense in the past when we were surviving in the wild, but today it certainly doesn't. In fact it's counterproductive and destructive.
This picture perfectly illustrates what I mean. If you blindly assume what other people do is correct without assessing it yourself, it's guaranteed you'll end up in trouble.