>...if you’re truly on $3k/month because you are unable to work, you have a serious disability.

They're most likely talking about Social Security Disability Insurance (SSDI). The amount is completely unrelated to the severity of a person's disability.

The reason is in the name--it's insurance for lost wages. Workers' FICA taxes are the premiums they pay while working, and are based on wages as well. So basically: the more one makes while working, means the more they pay in while working, means the more they lose when disabled, which means the more they get paid by SSDI.