@quinn What even IS a billionaire?
Money isn't a physical commodity, like lumps of metal; despite what goldbugs and "kitchen sink" economists think, money is simply a measure of economic activity. If it stops moving it's worthless. So "owning" money is more about controling what it's used for than tangible property. Added complication: capital accumulates—money attracts more money, like mass giving rise to gravity.
So what we're saying is "how about we not let individuals control stuff".