Why, at least on some level, can’t people figure this out for themselves?

The problem is that they think they have figured it out for themselves. 

As humans our senses absorb signals but they also screen out signals.  The neuroscientists and psychologists tell us that what we do to navigate our environment is to create a really good sounding story that seems to accommodate our impressions.  Then we spend all our time accepting signals that confirm this story and rejecting those that contradict it.  We all do this even when we are made aware that we’re doing it.  It’s part of the programming, not a glitch.  

There are some strategies that can mitigate against limited perception, however.   One of these is to deliberately expose yourself to a wide range of opinions, especially ones that conflict with your own.  The theory is that these other folks are probably allowing signals in that we are screening out.   At the very least you will begin to understand the actual perspective of the people you disagree with rather than your own perhaps mistaken impression of what they think.

This approach will eventually land us in a sort of epistemic agnosticism but what’s wrong with that?  Certainty is the enemy of thought.   Saying “I don’t know” is the most honest statement we will ever make.