There are two forms of Social Security disability benefits, one paid from payroll taxes (SSDI) and one from general tax revenues (SSI, this one). I for one was not really aware of the distinction.
From my families experience, SSI is very hard to qualify for; it’s for when you have not worked long enough to pay into and qualify for SSDI, which is more lenient. I can imagine it’s rare comparatively; especially if you don’t associate with people who have never really had a job.
I don't think it's all that rare. It's all the people that grew up with medical issues that preclude work. It's just that until the internet such people tended to be pretty isolated from most of society.