For the obvious reason. Fiat is purely by decree and based on trust in redeemable value by the issuing body - a faith based money, because there's no intrinsic value. To bolster this faith, they use techniques based on simple human instincts to trick the unsuspecting mind. These fraudsters have been at the game a very long time and for most average people who equate gold and silver with 'value', it's easy to see how they look at modern 'gold and silver' coinage and think they also have value. For sure anybody who's got even the most rudimentary education will understand that there's actually no silver or gold in the coins, but such is the human nature of normalcy bias, the transition from real gold and silver coins to the new is much easier if they not only buy the same things, but also look the same as well. It's a very subtle mind job at play here that works.