“Ownership”
Under the Bush plan, when you retire you are mandated by law to use your private account funds to purchase an annuity substantial enough to keep you above the poverty line for the rest of your life. (I guess that whole letting people decide what to do with their own money bit only goes so far.) You’d have to figure that for most retirees that would run through pretty much the whole stash or at least the lion’s share of it. And when you die, that’s it. By definition, you can’t pass on this kind of annuity.