But you still need that money to keep up with inflation. If you have a billion dollars under the mattress and inflation is 3% you've lost 30 million dollars in

But you still need that money to keep up with inflation. If you have a billion dollars under the mattress and inflation is 3% you've lost 30 million dollars in spending power. That dwarfs anything you would spend on food yes. And the next year another 30 million, and the next year. So you need to invest to protect it from inflation, so inflation is relevant. The percentage they spend on food and stuff today is negligible. But you invest to keep it that way. Because once it becomes non negligible it's too late.