I think one of the reasons why schizoid personality disorder has such a poor prognosis even when we do get treatment is because therapy is really hard when your brain is hardwired towards isolation and not letting people in.
For schizoids (especially those for whom trauma was a factor in developing SzPD), having even small amounts of emotional intimacy in a regular friendship or acquaintanceship can feel extremely overwhelming and unsafe. Another person finding things out about you can make all future interactions with them feel suffocating.
With how most people struggle with opening up in therapy, combined with how difficult any kind of openness can be for schizoids, you can probably see why this would be an issue for us. But one thing I think many MH professionals I've seen haven't understood is that sharing literally anything, even things that seem "small", can be very difficult for schizoids. It's not just the big vulnerable things, it's anything that's perceived as "personal".
Sure, I have trouble sharing the deep rooted personal issues that are at the heart of all my problems. But I also have trouble sharing relatively simple things, and it can cause major problems in therapy.
For example: the first time I did EMDR, I could not for the life of me give my therapist an answer when she asked what "safe space" or "safe image" I had picked to think about after the processing, or during if I got triggered too badly.
She assumed that I just couldn't think of anything (and I guess that was partially true because there isn't really anything that makes me feel a strong sense of comfort, since I don't really have that kind of emotional range), but no, I just couldn't voice anything I was thinking of because just telling another person about the kind of imagery I find calming was too much.
She ended up picking something for me I didn't even really find calming, so you can imagine how well that session went. But I can't even fully blame her, because she had no idea what the actual reason for why I wasn't telling her anything was.
But in so many other contexts as well, I've had therapists do that thing where they try to tell you their analysis of you and they've just been completely fucking wrong. And it's not necessarily because they're bad at their jobs, it's just because I didn't tell them anything, and I've mastered the art of saying just enough that the other person is satisfied without giving anything away.
It makes the whole process very frustrating, because on one hand I know I have significant mental health issues that are making my life miserable, but at the same time I am viscerally uncomfortable with someone else knowing what goes on in my head.
Just another reason why psychology courses basically ignoring "rarer" conditions (that aren't even that rare to begin with) is unacceptable. These things would be much easier to navigate if the professionals I see knew what my diagnosis actually meant.