There are a number of reasons for this. Where you see the price of gold changing it is invariably the value of the dollar against the value of gold that is actually changing. Not the value of the gold.
The value of gold has not materially changed for the past 50 years or so. You can still purchase the same material goods and services with an ounce of gold as you could 50 years ago. This does not apply to the value of the dollar of course. It takes a lot more dollars these days to buy the same goods you bought 50 years ago.
The question, for investment purposes then, is what do you invest in? Dollars or gold?
Investing in dollars leaves one open to the manipulative fluctuations resulting in the dramatic variations of value in the dollar and the devaluation caused by the continual printing of more dollars (we are talking US dollars here but the same principle applies to many currencies around the world) to fund debts such as the federal government debts as well as public and private debts.
Gold however, remains consistent and in fact does not increase in value in terms of purchasing power. Rather the value of the dollar decreases, in the long term, against the value of the dollar. Once upon a time 20 dollars would have bought one ounce of gold. Now it takes between 600 and 700 dollars and this is quite likely to go higher as the manipulation of the dollar continues.
However what you could buy for 20 dollars 50 years ago will now cost around 600 to 700 dollars. Interesting! This tells one immediately where to put one's money if you want to retain the purchasing power of your assets.
Gold coins, bars, or ingots, all are suitable instruments of investment for the long term haul. Gold is interchangeable for goods and services. It can be used as a currency and often is. Gold is not dependent upon any promise. One simply hands over the gold or receives the gold.
The value of gold is determined by its usefulness rather than by any manipulation (extremely difficult with a sold metal which may explain why the gold standard was removed in 1971 enabling the dollar to be subjected to more manipulation.
According to James Turk in his article "8 Things Everyone Should Know About Gold", James Turk,
"Though central banks do not control the gold market, they can influence gold’s price. Importantly, their influence is diminishing. Central banks have been dishoarding much of the gold in their vaults, so they now hold a relatively small part of the aboveground gold stock. After the Second World War, about 68% of the aboveground gold stock was in the vaults of central banks. It’s now about 10%.It seems therefore that the more divorced gold is from the dollar, the better it fairs and the more stable and therefore attractive it will become.
Less gold within their control means that central banks have less influence on its price, which is one of the reasons central banks are no longer the factor they once were."
In a competition of gold vs the dollar investments it seems that gold indeed wins hands down!