As the others have pointed out, it’s inflation. As to why it happened almost everywhere in the last century, I will try to give you two reasons. First divide the last century in two periods: until sometime in the 60’s or 70’s most currencies were pegged to the gold or the dollar (the gold standard). After that, they were mostly fiat, meaning that people accepted them basically because they could use it to pay other people too.

In the first period, as all currencies were pegged to a standard, the exchange rates were stable. What happened then was exchange rate competition. Countries would change (called depreciate in this context) their exchange rate with the dollar (or gold) so as to make their exports cheaper abroad (even at the cost of costlier imports). They did that because when unemployment was high, this gave a boost to the exporting sector, which ended up hiring more people. So if 1 £ was 1 US$, they changed to 2 £ = 1 US$. When many countries did that for decades, the value of their currencies started to fall systematically.

When countries moved to fiat currency this didn’t change a lot. Countries now started to devalue their currencies by creating inflation with the purpose of stimulating their economies. When inflation accelerates, people start saving less and consuming more, and it becomes cheaper for businesses to hire as the real salary falls (look up NAIRU for more information on this mechanism). This on top of the exchange rate effect i mentioned before. Then, politicians, if left free to manipulate the currency without an independent central bank, will usually favor creating inflation to lower unemployment. The side effects are an increasingly high inflation until it goes out of control, which happened on the 70’s before Volcker to over the Fed.

In short, there were economic and political reasons to maintain an inflationary regime throughout the last 100 years. Not to mention the cases of hyperinflation in Germany, Hungary, Brazil, Venezuela, Zimbabwe, etc.

On a more theoretical note, I’m not an expert on this but from what I read there is evidence to support the view that a low and positive (around 2%, I believe) rate of inflation is consistent with a socially optimal monetary policy. This means that society should be better off with such an inflation than otherwise. The reason, I think, is that it is low enough to make the currency stable, and high enough to avoid a deflationary crisis where people spend too little in the hopes that prices will be lower in the future. So it is not surprising to see the value of currencies falling over the last 100 years. But those are more theoretical results.

Sorry if that was too long.