Being autistic is a part of a person. It can't and won't ever change and, for many people, they wouldn't want it to if it could. It is permanent. Autism (I believe, I have no direct experience) has many positive things about it as well as the negatives and those with it are often proud to be who they are. It is, in my mind, a disability but I'm willing to accept that, for more high functioning individuals, that's only because society makes it that way.
Schizophrenia, Personality Disorders, Anxiety Disorder, Bipolar... All MH problems that are (or can be) permanent. Also... I don't think many people like having an actual disorder or even want it. I have adhd and another undiagnosed LD and I just learn to work around it and accept it. I'm sure it's the same for someone with bipolar, you learn to take your meds and soldier on.
You're in serious denial if you think people with disabilities actually want to have disabilities. You think an autistic person likes being overloaded so that wearing a T-shirt feels like having spiders crawl all over their arms and neck? You think they like or are proud of once again accidentally pissing someone off or hurting a close friend because they didn't know how to show support for their friend in a way that would help?
A mental health problem is not an intrinsic part of a person. It can and in many cases will change and few people want to have their condition. It is often not permanent. There is nothing positive about being mentally ill and nothing to be proud of in it. It is not a disability, though it can be disabling, just as a physical illness can be.
But it is. As kids with LD's or delays grow up their conditions can lessen in severity and they can even function out on their own. Stress can exacerbate them, and routines can make them recede. It's no different than physical disabilities. A kid with CP might be able to run some days and not others. It's not a flat unwavering line. It's really no different than MH problems.
There's nothing to be proud of in having a disability. Don't be naive. It's frustrating, it only works to your detriment, and it does nothing to help you. The name "disability" makes it clear that you are not functioning physically or mentally as well as everyone else. There's plenty to be proud of in overcoming a disability and figuring out a way around obstacles, just like there is in MH problems.
The problem isn't that disabilities are bad, it's that people look at the person with MH problems negatively. The same doesn't happen with someone with cancer. People look at a person with cancer and go "Oh, the cancer is terrible, but the person is still an individual who likes trains or whatever".