The goal here is to protect people against risk: of unemployment, of health emergency, of outliving one’s savings, and so on. For a risk-mitigation scheme to work, there are a few things that are necessary. One is that people actually be covered. This is something you can never have with a private system (unless it’s regulated to the point of being essentially public), since charities get to pick and choose whom they want to help. As Konczal says of private agencies before the Depression, “They were also concerned they’d lose their ability to stigmatize—or to protect—various populations; by playing a role in determining who wasn’t deserving of assistance, they could shield those they felt worthy of their support.”
Another thing you want is the assurance that the system has the financial capacity to actually protect you in the event of a crisis. That’s why you don’t depend on your neighbors to rebuild your house if it burns down. Besides the fact that they may not like you, they probably don’t have enough money—especially if you lose your house in a fire that burns down the entire neighborhood. As I’ve said many times before, there is no other entity in the country—and not really one in the world—with the financial capacity of the federal government. Even state governments scramble to cut benefits when push comes to shove, which is one reason why some states provide Medicaid coverage to almost no one.