Because economic output is growing. If the money supply grows to match, then each dollar has a stable value. If the money supply doesn't grow, then each dollar becomes worth a fixed fraction of the economy, and therefore the value of the dollar grows as the economy grows. (It's essentially the same as saying, if the amount of liquid increases, we want the number of liters to also increase, so that a liter can represent a fixed amount of liquid.)

Why does that matter? Well, let's say you own money on your house. And let's say that each of those dollars is worth more money, year after year, for a thirty year mortgage. That's going to become a problem, a big enough problem to crush you.