I realize that to the beginner gold prospector this may sound wrong. After all, if they know there's gold there, didn't they get it all already?
The answer of course is, no they didn't. For several reasons.
First, it has just never been practical and cost effective to try to get all the gold. So early miners settled for the easy gold. They had to eat, after all, so when it got too difficult and time consuming they just moved to another place.
Then, during the great depression in the U.S., many people turned back to gold mining to try and eak out a living. But during world war II, the U.S. government made it illegal to own gold, which put an end to even the most successful mining operations. So even though they knew there was gold, they had to leave it because there was nothing they could do with it.
The gold that early miners left as impractical is still there for the recreational gold prospector to retrieve. Maybe you won't get rich from it (but hey, who knows?) but if you know what you're doing at all, you will find gold.
Besides, we have new tools and technologies today to help us find and retrieve the gold that early miners could not even dream of. Tools like metal detectors, suction dredges, power sluices, outdoor vacuums, dry washers, and more. These tools give the modern gold prospecting enthusiast a huge advantage over the prospector of old.