Why would a person facing starvation want to give up his remaining supply of grain in order to get some money? What good would the money do him? He wanted life, not money, and grain offered life. Because the money had “failed,” it had fallen to almost zero value. Thus, in order to buy food, the people had been forced to spend all of their money. Now they were without food or money.
They lost everything and were willing to sell their bodies for food in the way of slavery. In fact, grain became the new form of money, although the Bible doesn’t say this explicitly. What it says is that everyone was willing to trade whatever he had of former value to buy food. But if some item is what everyone wants, then we can say that it’s the true money.
Because the money had failed, it had fallen to almost zero value. So, in order to buy food, the people had been forced to spend all their money. Now they were without food and had worthless money.
Were the Egyptians foolish? After all, those cattle and horses were useful. But animals eat grain. The grain was too valuable during a famine to feed to animals. All that the animals were worth was whatever they would bring as food, and in Egypt, the meat would not last long. Dead animals in a desert do not remain valuable very long. Why not trade animals for grain, which survives the heat?