We don't appreciate how difficult it can be to employ these marginally capable workers. Keeping these people on staff at a small company raises health and operational insurance costs pretty dramatically, so it can be hard to justify keeping them on staff, even if you wanted to.
Disability is also pretty difficult to get, despite how the news makes it out to be easy. My father had multiple heart attacks and cancer, and he's still fighting to get on disability. And when he does, it will take years to come into full-effect.
Unfortunately, they were also the first laid off when automation was installed, and afterwards they were competing for few jobs at the same (regional) low wages with "normal" people.
FWIW most of the people I've met on disability probably deserved to be on there, including people I've met with severe mental illnesses. I'd posit there's more of an empathy gap here than anything.
You need more, less-biased samples!
Most of the patient populations my relatives are referring to are poor, inner-city minorities, who the average HN reader is unlikely to encounter in day-to-day life, so it's entirely possible for the average HN reader to only meet legitimately disabled people while still having a sizable fraction of people on disability collecting disability for bogus reasons.
By the way, I think it's socially undesirable for people who are not really disabled to go on disability, but for many of them it is a rational response to their circumstances.