Why are US mint golden eagle coins more expensive?

Because they have a coin value along with the gold value. Coins are almost always worth more than just scrap value if they are in decent condition. With the pre-1933 gold coins many of them were melted during gold confiscation during FDR's time in office and as such are rarer and more desirable than generic gold bullion or European gold. But usually coins made by a reputable mint (US mint, Royal Canadian Mint, the British Royal Mint, etc.) are worth more because the coin has collector value when compared to simply a precious metal round or bar made by APMEX or other private companies.