I think of the economy as a giant lazy river. A large quantity of life-critical fluid circulates endlessly, in a wide loop.
Most people spend 9 hours a day walking down to the river (commuting), filling their vessels (working), and carrying them back home (commuting). Some people also have an at-home cistern that they pour their vessels into (savings) that provides a buffer beyond the home's immediate needs.
A smaller number of people have gigantic reservoirs (rich). They don't go down to the river themselves to fill up. There's no way they could carry enough with their own buckets to fill even a fraction of the volume of their reservoir. Instead, other people pour in a little every time they make a trip to the river, perhaps as part of a deal to supply larger buckets, or shoulder yokes, or carts.
The big private reservoirs remove water from the normal water cycle. When people use the water, it flows back into the river somewhere downstream. Sometimes different stretches of the river experience droughts or floods, and that is what those little at-home cisterns are good for. During a drought, the river narrows, and not everyone is able to fill their vessels all the way. And those giant reservoirs--which aren't actually being used, but instead protected against leakage, siphoning, and evaporation--each slightly reduce the water flow downriver.
During a drought, some people may resort to begging water from a private reservoir owner. And if they have stout legs and shoulders, the owners agree: you can take a little of my water, if you promise to give more of it back later (loans). Sometimes, even in good times, someone will get water from a reservoir, because the reservoir is closer, and they need more than they can personally carry, and they promise to put more water back into the reservoir later (investment).
But as the water is locked up in private reservoirs, the river slows to a trickle, and only the rich can drink without pledging some of their future water-carrying capacity to someone else. The reservoirs grow larger; the river shrinks smaller.
Water that is used returns to the river. The poor and middle class need the river, as it is their only way to drink without mortgaging their future away to a rich person.
You might already see the problem here. The river comes from water that has been used and recirculated (spent!). When someone uses reservoir water, and pledges interest or investment return on it, they are committing to put more of the circulating public river into the unmoving private reservoir.
The only way to refill the river from the reservoirs, and keep it in circulation, is for their owners to use more water. Or for armed river rangers to order them at gunpoint to drain some of it back into the watershed. There is a little natural rainfall, and a little from people melting glaciers a little bit at a time by holding chunks of ice under their armpits, amounting to about 2% or 3% of the known volume of liquid water, but all those existing reservoir pledges have already exceeded that influx.