Because unless it's been previously vetted, your brain can't tell if a fact is correct or not on sight.
But your brain instantly knows what pisses it off.
Think of your brain as a nightclub. Your knowledges, facts, and ideas are the patrons. Eventually, you develop a staff of bouncers (judgement, bullshit meter, call it what you will). As you grow and learn, these bouncers learn what Ideas are ok ("Human beings can't fly"), what Ideas are harmless ("wouldn't it be cool if we could fly?") and which Ideas will fuck it up for everyone ("I'm Peter Pan, motherfuckers! Where's a balcony? I need a good leaping spot!").
As time goes on, you accumulate not only regular Facts who have been vetted time and again, and who have friend networks that vouch for them. You also accumulate ideas non grata, ideas that showed their ass, got kicked out, and aren't allowed in your head.
When one of these ideas shows up, it sets off the emotion alarm. Brain chemicals kick on a big "fuck no!" sign. Your more violent and righteous ideas start yelling.
Now, if you've trained your bouncers to investigate before attacking, the least that happens is that you stop and see what this asshole idea is up to, and what other ideas they're with.
But often you don't have that kind of time or inclination, so you just batten down the hatches and tell the entire group to fuck off, all because they're hanging with the asshole ideas, making them asshole ideas by affiliation.
My brain is full of gun nuttage and use-of-force laws, with networks of affiliated facts going several layers down.
A few weeks ago, one of the latest memes had a list of hashtagged names of unarmed black men killed by police in America with "= No Conviction" next to each name.
That band of Ideas got stopped at the door to the club of my brain. Because just the facts near the door and the ones hanging out near that door know that two of the names on that list were legitimately killed according to every known use of force law currently in effect.
My bouncers turned those two lines away right at the door. I had neither the time nor inclination to wake up the nuances I know were in effect. So the rest of the names were turned away too.
Sitting in the back behind all of those ideas was, "systemic racism in American legal and justice system."
And my bouncers looked at it and said, "you realize we're going to go through this every time you go out with those lying assholes, right?"
And it went, "yeah, yeah, they're a lot more useful with other crowds, though."
Which is true.
What happens if that idea went to, say, a sociologist's door? Someone who's expertise was broader but shallower? Who knew about more cases but less about the nuances of each individual case?
In that mind, systemic racism would roll deep with the entire pack of ideas behind them and the bouncers would never say a word. Any ideas that would object wouldn't be up to vet or object, assuming they had ever visited that mind in the first place.
In which case, the first idea to refute or deny a member of systemic racism's crew would be in for their own step-by-step vetting to get into a new brain.
This happens with all sorts of ideas, in everybody's brain.
When Kapernick took a knee, a fuckload of people saw "systemic racism in the American Legal and Justice System" hanging out with "Make a conscious display during the national anthem."
And a LOT of people's brains clanged shut and said, "how dare you bring THAT asshole idea here!"
They still are.
The point I'm making is to take at least some time to let your bouncers vet the occasional idea, even if it runs with an asshole crowd.