"...why have certain Federal Workers like the Post Office opted out."
I tell you, that's got a down side. A friend of mine's husband got leukemia (he recovered eventually) but because he worked at the Post Office he had NO disability coverage. He was too sick to work, but not sick enough to retire, so he was stuck for about a year with no income. I suppose what he SHOULD have done long before was purchase an individual disability policy, but I think they are pretty expensive.
Do you think it would have been any easier to get Social Security?
I have a close friend that had a severe head injury. She was so screwed up that after 3 months in a hospital and rehab that she had to wear a football helmet in her own home to protect her from further injury due to seizures.
She lost all driving privlidges, she wasn't allowed to cook.
Getting classified as disabled so she could get her Social Security benefits at 54 was not easy.