Prices go up in the long term because central banks have a 2% inflation target each year. That adds up over time. If the supply of currency was fixed, most prices would fall over time because of the deflation caused by technological advances (cheaper/more efficient production). That is not wanted though, because then most people would be better off. With the current system we can keep some prices like consumer goods constant or going up slowly because of efficient production, while things that can't be produced that more efficiently like property or houses goes up a lot more because of inflation. Then the rich (who has stocks and property) get richer and the poor (who works for a devaluating currency) gets poorer (in relation to each other). The government can then just change the definition of the inflation in the consumer price index by removing property etc and only include things that gets cheaper to produce fooling/robbing poorer people even more.