Economic theory can never be valid if it is detached from the influence of physical force. As the post about "power" alludes to.
All property is something someone can control, ultimately by physical force if necessary. For example, under common law you physically control your hat. If a thief takes physical control, the law may return it to you.
Your pile of gold locked in your house is controlled by physical force, and you can use guns to defend it, in many circumstances.
Less tangible property, like currency holdings, are backed ultimately by the physical force of the US. Only if the US sides with you do you actually own it. It could turn against you any minute and take it all - seize your bank accounts, etc. Those who vie for control of the US often do this to each other, but if you're poor it's not worth their while to do it to you.
The rich get rich by exploiting this system and trying to control it. The poor don't try to control it, that's how they get poor.