Because it's a natural thing to do. It's very tiresome to watch yourself all day to make sure you're not making any assumptions. We hear "CEO has relationship with employee" and our mind immediately goes to work fabricating a mental image of that. And if there's something our mind is really good at, it's filling in the gaps, i.e. assumptions. So when we continue the conversation, we use that mental image and without even thinking use the word "she" when referring to the employee. Is this insensitive? No, it's just sticking to patterns, which is the modus operandi of our mind.
I think the parent is descriptive for many people, but that's a different issue than prescriptive. We can do much better:
> Is this insensitive? No, it's just sticking to patterns
IMHO, that's the cause of most discrimination. People aren't intentionally discriminatory, but not having experienced it themselves they suffer from the blinders of their own perspective (as humans do), are unaware of the consequences of their actions, and are doing the very human thing of downplaying the magnitude of others' problems. As Mel Brooks (IIRC) said, 'Tragedy is I stub my toe; comedy is you get eaten by a lion.'
That's why people talk about being 'woke' or becoming aware of their 'privilege'. They weren't intentionally insensitive before, but they didn't grasp what they hadn't experienced, and then one day the blinders come off.
> the modus operandi of our mind
The modus operendi of our minds can lead to all sorts of horrible things, from murder to war to theft. We can and do use reason in order to do better, including by learning the pattern of not making assumptions.
> it's a natural thing to do
It's natural to follow patterns, but the pattern isn't natural; it's just one arbitrary pattern we learn. We can learn other patterns, which is how we change and grow. Whole societies learn new patterns; democracy and universal human rights didn't exist until the 18th century (roughly speaking); for all of human history until maybe 50 years ago, women were 'naturally' though of as qualified only for raising babies and maybe some nurturing jobs. Now they fly fighter planes off aircraft carriers and are a majority of new lawyers (IT hasn't figured it out yet, apparently).
> It's very tiresome to watch yourself all day
If you think that's tiresome, imagine facing endless discrimination everywhere you go, from family, co-workers, employers, the person on the bus, the movie, the book, the Reddit thread, etc. Imagine facing the prospect of experiencing that for the rest of your life.
Anyway, we don't have to watch ourselves all day. Just learn something new, and pretty simple. I learned some new Vim techniques recently and now I don't have to watch myself all day, I just use them. I learned to say 'they' awhile ago, and it's now second nature - Vim was harder.
