It seems to me that most people are able to think in terms of probabilities (or degrees, more generally) most of the time. Those people need to split the world

It seems to me that most people are able to think in terms of probabilities (or degrees, more generally) most of the time. Those people need to split the world into safe/dangerous, good/bad, likely/impossible. Also, they tend to not gradually update beliefs based on data the way Bayes' Theorem would dictate, but will often go directly from one extreme to the other. Some will tend to add an additional level of "unknown", but even in that case, they tend to go for "as likely as not" 50/50 odds, not 80/20, 90/10 or 99.9/0.1, based on evidence. Maybe doubt/uncertainty is too much of a cognitive and emotional load? For topics that are also seen as essential symbols of some social group (religion, culture war side, nation), the number who will admit shades of gray fall even further, of course.