Because the nature of inflation is to devalue money. For example, let's say you put $10k under your bed to buy a car for your kid when they turn 18, and let's s

Because the nature of inflation is to devalue money. For example, let's say you put $10k under your bed to buy a car for your kid when they turn 18, and let's say inflation was at 2% for those 18 years. Now the 18 years have speed buy and you go grab your very old stack of $10k from under the bed frame to celebrate your kid leaving the house. When you get to the car dealership, you find the equivalent car you could have purchased now costs $14,282. Your money has lost almost a third of it's value just by inflation.