Here’s another one: Capitalists want to pay their workers as little as they can get away with. But they want workers to spend as much money on their products as possible.
So any given capitalist will underpay their own workers for short-term profit and expect workers to spend extra money on their products. But those workers’ employers do the same thing, so those workers don’t have enough spending money left over to buy their products. At a large scale, this is not a sustainable system.