Tomato tomato. It's insurance on a societal level - the guarantee that your children, for example, will still be able to afford a medical policy the same as everyone else can even if they lose the genetic or developmental or whatever other lottery.

There's no divine directive that states that risk sharing must be done by voluntarily joining a pool run by a private entity that's priced uniquely per individual. That's merely one way to go about it.

To be fair it's the only sensible way for a private entity to go about it in the absence of legislation that prevents others from being more efficient. In the case of the US specifically, as long as everyone is forced to play by the same "inefficient" rules the free market will continue to work.