Because it's original work they contributed for free. Lending others that kind of expertise and time, just that it get's used by a machine learning algorithm, which aims to reproduce this, without giving it back to them or the community in a similar free manner, feels violating.
Apart from that, creators feel ownership over their content and it feels wrong not to be asked what happens to it. (Although those probably wouldn't – or shouldn't – use SO anyway, as their content gets commercialised anyway by giving it SO for free.)
As far as I know, I am neurotypical, so mind that when reading my comment.
Regarding the title of your question:
Because neurotypicals are the majority of people. As usual, majorities set norms and actively or passively decide upon common concepts like ethical or societal rules.
Regarding the issue you described:
Even though it might seem exhausting, I think it's important to see people – regardless whether neurotypical or not – not as a homogeneous mass but as a highly variable mixture.
Sure, even then there are common rules like don't punch people. But those are the broad and general ground rules.
Then, there are the nuances, which can be highly individual.
In your example, regarding the punctuation, use of grammar as well as verbosity of replies, I would react completely different than your friend. I wouldn't mind any of those things, which seemed rude to her. Okay, maybe I would feel like I weren't given the attention I hoped for if, after pouring my heart out, I just get a simple "K." as response. That would make me sad, because I was hoping for compassion and a dialouge dealing with the issue.
But apart from such things, that would probably be totally fine for me.
One behaviour and two very different reactions due to two different people.
As such nuances are often individual, it's probably best to explore what kind of behaviour the other person would feel comfortable with and with which behaviour they don't. Repeat that process with everyone in your life, who you would also like to keep relations to.
That's also related to a thing known as role-behaviour in psychology, which also applies to neurotypicals. When I talk to my superior at work, I behave differently than when I am at home with my wife. Then, I also behave differently with my friends. And among the friends, I learned enough about some, to know what and how I can say something to them and from which topics or phrasings it's probably best to steer away.
Imagine someone would sneak into your home and steal your shoes, socks and underwear just to get off on that or give it to someone who does.
Wouldn't that feel wrong? Wouldn't you feel violated? It's the same with such AI porn tools. You serve to satisfy the sexual desires of someone else and you are given no choice. Whether you want it or not, you are becoming part of their act. Becoming an unwilling participant in such a way can feel similarly violating.
They are painting and using a picture of you, which is not as you would like to represent yourself. You don't have control over this and thus, feel violated.
This reminds me of that fetish, where one person is basically acting like a submissive pet and gets treated like one by their "master". They get aroused by doing that in public, one walking with the other on a leash like a dog on hands and knees.
People around them become passive participants of that spectactle. And those often feel violated. Becoming unwillingly, unasked a participant, either active or passive, in the sexual act of someone else and having no or not much control over it, feels wrong and violating for a lot of people.
In principle that even shares some similarities to rape.
There are countries where you can't just take pictures of someone without asking them beforehand. Also there are certain rules on how such a picture can be used. Those countries acknowledge and protect the individual's right to their image.