Because the things “everyone knows” just might be completely wrong – and leave your book or rant or whatever open to ridicule, impromptu flying lessons and other undesirable consequences.
See, it’s a big, complicated world out there with so much to know that nobody can actually grasp all of it, so we all tend to optimize: we’ll take as given something that doesn’t particularly interest us at the time and plan to remember it long enough to pass that exam or whatever, but what happens is these unexamined things tend to hang around and form the basis of a lot of assumptions, especially when they’re things we’ve learned early on.
The result is a whole lot of people believe without question the things that were presented to them as fact by people they trust (who in turn were told these things by people they trusted, and so on ad infinitum) and never sit down to examine them.
After all, everyone knows the Sun and all the planets revolve around the Earth, right?
Well, they did until someone questioned the assumption that the Earth was the center of all things. Then the usual result of having your assumptions questioned happened – it’s not pretty, particularly when someone’s faith or their entire notion of who they are is based on them.
In that light, a lot of the fuss and tantrums we’re seeing lately (coughHugoAwardscough) start to make a lot of sense: up until very recently the assumption of those who actually knew what a Hugo Award was was that it was a prestigious award which almost always went to people whose works deserved the honor. Then along come the Sad Puppy campaigns blithely challenging this perspective and – horror! – suggesting that the Hugos might have become a kind of almost incestuous club (Let’s face it, when you look at the nominee and winner lists, the Best Editor – Long Form was an incestuous club from the start).
What I mean here is that the group of people nominating and voting was sufficiently small that everyone knew everyone – or mostly – and most of the convention SMOFs knew each other so without there being any need to invoke conspiracy, works, creators, and editors who didn’t fit the mental model most of that relatively small club (fewer than 500 people in the first year that the data is publicly available) held simply didn’t get considered.
To use a somewhat less contentious example, I often see rants from my historically knowledgeable friends (and have been known to indulge in similar rants on topics where I’m reasonably knowledgeable my self) about books where the author assumed that their knowledge about say, a man’s “ownership” of his wife in late Victorian England up to the level of being able to kill her, was as accurate as said author assumed it was (It wasn’t, particularly at the middle class and higher social levels). Or the lamentable assumption by any number of writers that the way people thought and acted and clothed themselves in the past was more or less the way we do things – if I had a dime for every draft/slush offering/fanfic I’ve seen where the woman in the fantasy or historical setting wears a bra and panties under the dress, I’d have a lot more money than I do now (corsetry is a historical field all by itself – the technology and creativity dedicated to uplift and control of feminine weapons of mass distraction is really impressive).
Quite simply, it doesn’t matter what it is, or what side of the political fence(s) you fall on, chances are you’ve got an unexamined assumption hidden in there and you should check on it. I can’t count the number of times I’ve read something then gone looking a bit deeper and found that what I read was third or fourth-hand, and the primary source was damn close to the exact opposite of the first piece I read.
If you know someone with more expertise in the area, ask them. Send the piece to friends of a different political persuasion (you do have them, right? With the exception of those whose ideology demands that all who believe differently are evil and must die, we’re all capable of maintaining at least a friendly acquaintance with people whose beliefs are very different than ours). Research for yourself – and look for primary sources where possible. Even if this means wading through legislative legalese.
You owe it to yourself to check that your assumptions aren’t a case of “the things you know that ain’t so”.