Remember that a lot of humans only see, basically, in two dimensions; things, people, and even concepts and ideas look more like cardboard cut-outs to them.
Things, people, and ideas don't appear to have MORE to them than what they see at first glance. 

That isn't to say that they literally see things as line drawings, or as "flat objects" floating in space, it's referring to a lack of ability to understand that there is much more to anyone and anything than one can see.

So when they see a person, whatever their EYES see is all they think is there, just like a cardboard cut-out. Their brains do not wonder ABOUT the person, what more there is to the person, because they don't think there IS any more to the person. They believe they know all about the person from what their eyes see, and also from what they themselves FEEL when they see the person. That's all there is, there just ain't no more.

They will do a similar thing when assessing a person's facial expressions, body language, and speech sounds. They believe they are correct about whatever they think the person is expressing, regardless if it's even CLOSE. It doesn't occur to them that they might be misinterpreting the person's expression, or reading it completely WRONG.
If they see or hear something the person does or says that they want to read as "ignorant", "stupid", "mean", or "crazy", they'll just assume they're RIGHT.
And if see or hear another person's social signals as "contemplative", "authoritative", "intelligent", or "in control", they'll just assume they're right about THAT, too.

Basically they just go by their presumptions about "who" and "what" a person is, and then they interpret everything the person does and says through that.

If someone tells them something about the person, they tend to just BELIEVE IT, for the same reasons. No wondering, no doubting, just immediate belief. No filtering anything through "values", "morals", or "ETHICS", no paying attention to their own biases or reviewing their own beliefs. No paying attention to their emotional reactions, or double-checking their motives.

No wondering "Is that even true?"
or "Why does this person seem to want me to LOSE RESPECT FOR this other person?"
or "Am I participating in hatefulness or gossip? Is this right?"
or "Why do I think I know ALL ABOUT another person without actually spending a LOT of time WITH that person?"
or "Am I reading this correctly?"
or "Did I interpret that right?"
or "Do I really know enough information to make a judgment?"

It doesn't seem to register that they would be very upset if people thought about them in such superficial, diminishing, demeaning, judgmental, presumptuous ways.

Nope, none of that. Just going along with whatever, just believing their own cloudy vision, their own assumptions, their own reactive emotions, their own envy, resentfulness, and also their own desires.
There just ain't no more, it's just not there.
So when a very large, "tough looking" man with dark skin turns out to NOT be a sports freak, but works for a Florist making bouquets and loves Chopin, and has three cats (instead of what, pit bulls, perhaps) they don't like it. Because they already thought they "knew" all about him, and now his reality is clashing with theirs. So they don't make friends with him, and they doubt his sincerity. They don't give him credit or respect for the things he does, they don't help his reputation, but they do tend to HINDER his reputation and make little sarcastic implications about him.

They don't like HIM because THEY were WRONG.

PLUS, they're envious of him, because he does not live within the same parameters that THEY live in. He's a "free agent" who doesn't seem to care about what other people "expect", and he doesn't seem to care what they think of him.
(And they also know that those qualities are attractive to those of the opposite sex that favor independent thinking and creativity over codependent conformity, so they're jealous, too.)

Maybe one or two of them will start cruel rumors about him in order to make themselves feel "good"... and make sure OTHER PEOPLE DON'T LIKE HIM. Not because he's a bad person, but because they don't want others to know that he's a good person.
When a very small, light-skinned, light haired woman turns out NOT to be a fragile waif who's most arduous task is sweeping a floor, or to be a submissive cheerleader who's greatest joy is baking cookies for "her MEN" to enjoy during their sports events (during which she reads all by herself, not minding at all that she's been excluded from her own husband's social life),
but she ACTUALLY fixes cars, runs earth-moving equipment, and works on airplanes and flies them, they FREAK OUT.
Some will find her "extra attractive", and will act like NO OTHER FEMALE HUMAN has EVER done any of those things... she's the ONLY ONE, EVER... OR, they'll do the same as with the large dark-skinned man, they will reject HER as a person, because she didn't fit into THEIR imaginary expectations of her.

Either way, she's a FREAK OF NATURE, as far as their concerned, either to be captured and owned, or to be suspicious of and despised.

In both cases, the large dark-skinned man, and the small, light-skinned and haired woman, the surprised reactions of other people were based SOLELY on the fact that they DID NOT THINK BEYOND the two-dimensional, cardboard-cutouts they had in their heads of them. They did not WAIT TO FIND OUT about either of them, they just ASSUMED that they knew all about them based on nothing more than their appearances.

No matter HOW MANY TIMES "Don't judge a book by its cover" is repeated, humans who only see in two dimensions and believe whatever they see and hear WILL NOT learn what it means. They can't, it's just not there. (That's why they fall prey to advertising and buy so easily and completely into politics and television shows. )

It might be hard to fathom, but this two-dimensional thinking is NOT limited to only the diagnosed "mentally ill", or those who have apparent "lower IQs". There are two-dimensional thinkers in every walk of life, and in very high positions of authority, and who have Ivy League degrees.
They don't see a sculpture and see the inspiration of the artist, or what the work is attempting to convey. It may not occur to them about the effort and skill the artist put into the work, or why they would even DO it. They just see it as "pretty, ugly, or boring", they only relate to it with "I like it" or "I don't like it", and maybe something about whether the sculptor was a man or a woman, or what ancestry they were. (That matters to them in a big way, usually; if the artist was a man, and they're male-biased, then the work is "genius"; if it was a woman, then that same work is "silly" or "not really that good", or "not as good as so-and-so's".
They'll think the same way regarding any bias they have, if they're female-biased, then it would be the opposite, and if they're ancestry biased, then it's a Great Work Of Art if the sculptor was the "correct" ancestry; if not, the same work will be criticized.)

Even harder to fathom, perhaps; a person can be a "brilliant" artist, musician, songwriter, scientist, doctor, healer, psychiatrist, inventor, veterinarian, healthcare worker, teacher, lawyer, coach/instructor, 'leader' in government/ social issues/ religion or spirituality, and yet be a "two dimensional thinker".

Having awareness of the bigger picture, or even the fact that there IS a bigger picture, or that there is MORE to the picture, is apparently NOT a requirement to achieve 'success' in this human realm, even if it SEEMS to be a requirement for one's specific occupation.

Two-dimensional thinkers don't know and don't care that there's more than they can see, or that they might not be seeing what's really there, or that they're interpreting someone or something incorrectly.