Short answer: Because gold nuggets are rare, and account for only 2% of all gold found worldwide every year.
Slightly longer answer: Most gold found today is tiny flakes or gold dust and requires vast amounts of ore crushing and processing to be refined. A rich gold vein will contain about 2-5 ounces of gold per ton of rock, and that's not including the overburden or non-gold bearing host rock.
Historically, most nuggets were molten down and refined for gold bars and gold coins. Even nowadays, large gold producers refine found nuggets to maximize gold production.
Increasing environmental protections and depletion of surface gold veins make hand dug nuggets extremely rare.
Natural gold nuggets come in various sizes, shapes and grades. These factors will have significat influence on the price. The larger, more attractive and pure a gold nugget is, the higher the price (and that exponentially; up to 10-15 times the value of gold spot price). This goes especially for nuggets from rare deposits, gold crystals and crystalline gold, nuggets over 1 ounce in weight and gold in quartz.
For our jewelry we use premium investment grade gold nuggets - nuggets that are especially clean (without host rock), beautiful 3 dimensional in shape with a shiny surface and a high purity (at least 20k). These are the hardest and rarest to find and all are hand selected by us.