The answers to your question are, I believe, these:
1. Some AS have overwhelming sensory issues, which can make it difficult to do any job that isn't inside a perfectly calming box. And not many jobs are.
2. Some AS have such severe executive function issues that holding a job, where following instructions and meeting someone else's expectations are part of the requirement, becomes impossible to do on a consistent, day to day basis.
3. Some AS have medical conditions that are common among AS, although not part of it, that make working difficult.
4. Some (most?) AS have a processing speed issue AND difficulty staying on task for lengthy amounts of time. Ever wonder why the IEP's for bright AS kids often include shortened assignments and longer test taking time? In some jobs you can get away with it, in others not.
My goal with my son is to channel his strengths so that he will be productive, and able to do work that relies on his gifts while not being severely burdened by his difficulties. He's way at the top of the spectrum, and I have no doubt it's possible. But this isn't true for everyone with AS, just as it isn't true for everyone who isn't AS. The world of work values other skills, by and large, far more than raw IQ.
My AS husband has a good career, as do I (but I'm more NT), and we are not on disability. But I see enough to understand how someone gets to that point. It's difficult to explain, but I have that compassion. I hope you will find it, too.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).