Because the conceptual difference doesn't exist for about 99.9% of people.
Also, most languages have no such distinction. For example in my native language Hungarian we either use the English word "gender" or say (the equivalent of) "social sex" and "biological sex".
It's a niche thing. I understand that some people feel sensitive about sex/gender or the differences between transsexual/transgender/transvestite/queer/crossdresser/drag-queen etc. But for the vast majority of the people this just isn't a topic of interest.
Around 0.3% of the population in the US can be identified as transgender, so that knocks it down to 99.7%. Then there's other factors, where people don't adhere to any particular binary gender at all, which erodes that even further.
When you include the full spectrum of LGBTQI you probably have to cut that down to no more than 95%, possibly less.
When you say "for the vast majority" you presume they operate in isolation, but they don't. Most people will end up knowing at least one person that doesn't conform to the traditional gender patterns, which means it's a problem everyone faces.
I was talking about the T (since the question was how come someone doesn't get that sex != gender). The LGB part is a lot more, some estimates are even around 10%. The "gender issue" is almost 2 orders of magnitude smaller than the sexual orientation issue.
So that's the explanation why most people don't "get" or even think about the distinction, but they know about gays and lesbians.
I've known three, and I don't have an awful lot of friends and acquaintances. I also don't go out of my way to only talk to LGBTQI folks. (I do, however, go out of my way to talk to people interested in technology and especially computer security.)
If you pool the various genetic causes of intersex conditions (neither unambiguously male nor unambiguously female) it's about the same percent of the population as natural red hair. Should we ignore people with red hair?
I'm not saying we should ignore them. Just that for most people the two concepts "sex" and "gender" aren't separate, and trans people are simply regarded as quirky. For example there are people born with 6 functional fingers per hand but we still say that humans have 5 fingers per hand and don't say that humans have anywhere between 0 and 8 fingers per hand.
Treating sex and gender separately makes it seem like they just accidentally coincide in case of 99.x% of the people.
One needs a very special kind of schooling and/or social circle to even imagine there's a difference. It seems to be very limited to (some) Americans.
Some people suffer from phantom limb syndrome. The pain is real, but the limb is not. We've not yet reached a point where the politically correct thing to do is to pretend otherwise.
Everyone understands that in the vast majority of times we use man/woman, he/she, or him/her, the intention is not to specifically draw attention to a person's reproductive organs. It doesn't take special schooling to detect this distinction, though it does require some thought to figure out what to make of it.
There would only be PC pressure to pretend a trans man has a penis if "having a penis" were a vital component of being a man, which it is not. In any case, the circumstances under which one finds themselves having to confirm the presence/character of another persons genitals usually either fall outside of or supersede the jurisdiction of "political correctness".
gender = {biological sex, social structures, gender identity, ...}
There's no reason to assume that the author has any ill will toward the LGBT community. It's a misunderstanding that people are (with good reason) very passionate about, but something that the author probably hasn't given much thought to.
It's not a clear distinction. It's easy to point to, say, trans people, and cite them as an example of people with differing sex and gender. Yet that's not quite true: a trans person who has undergone hormone treatment, for example, is not quite their birth-assigned sex.
As fluid as people wish it was, certain gender identities highly correlate with certain sexes. To be male sexually and female behaviorally is abnormal, statistically speaking.
This gender =/= sex is a recent concept invented by mankind.
The concept has only recently become popularized in the west, but the reality is not new that societal upbringing, reproductive sex, and mental self-image, while strongly correlated, aren't intrinsically linked.
That is a reality I do not agree with. Strong correlation indicates the high possibility of an intrinsic link, unless evidence states otherwise.
Below is a true story of a man raised as a woman, but unable to complete the sexual transition despite cultural upbringing, injections of testosterone and even testicular castration.
There is evidence in the form of the existence of transgender people. Some societies (as noted above) have been aware of the lack of an intrinsic link between apparent reproductive sex at birth and mental/social gender for quite a while, and analogous behavior is observed in some animals.
Just as you can't conversion therapy someone out of being transgender, David Reimer is evidence that you can't conversion therapy someone into being transgender.
Just because someone is born male, and raised as a boy, doesn't mean they'll agree once they can decide for themselves. They probably will, >99% of the time, but not always.
>There is evidence in the form of the existence of transgender people.
The link I describe isn't a link that ALWAYS holds. The link, like all links in science, is just a causal correlation.
So in short, being of the male sex doesn't guarantee that you will be of male gender, but it makes it highly highly like you will be of the male gender. That is the intrinsic connection and link I describe.
How it is so many people don't grasp this is beyond me.