Simply because gold has a long hostory (the longest), of stability. Even in times of war gold was seeked and pretty stabile. All the bank and countries have gold reserves and the fact that you invest in gold mean that you still have that money in the future. If there's a price inflation the gold is still gold. You can't buy the same ammount of food from you could have 10 years ago with 1000$ but you can still buy almost the same things (more or less) if you would have bought 1000$ worth of gold 10 years ago and sell it now.