This may sound strange, but there's simply too much gold. That is, there shouldn't be gold here at the Earth's crust - it all should have been sucked deep into the core long ago. For that, we can thank asteroids.
Gold is one of the precious metals, along with platinum, silver, palladium, iridium, and a bunch of others. While these metals do have a few industrial uses, they're mostly valuable simply because they're rare and they happen to make for pretty jewelry. (That may be a slight oversimplification.) The strange thing isn't that these metals are rare, but that there are any of these metals at all.
The reason for this is that the precious metals tend to be attracted to iron. Over the lifetime of our planet, the naturally occurring precious metals should have been sucked towards places with high concentrations of iron. That means all the way down to the planet's molten iron core. And while it isn't exactly impossible that some of these metals would simply be left behind, that could only account for about a ten-thousandth of all the existing precious metals.
Now researchers at the University of Bristol might have found a solution. They ran some high-precision tests on rocks found in Greenland. These rocks date back about four billion years and are the remnants of asteroids. The nature of the tungsten isotopes found within these rocks support the idea that asteroids carried a second round of precious metals to Earth billions of years ago, long after the original metals had been subsumed into the core.
So then, it wasn't just gold that people spent lifetimes chasing after - it was space gold. It's also an interesting reminder of how Earth is never truly isolated from the rest of the universe. Of course, that idea is hardly in dispute - it's looking increasingly likely that asteroids also brought water and the building blocks of life to Earth - but this is a unique case in which ancient asteroids helped shape our recent history. After all, without this space gold, what the hell would be the point of playing Yukon Trail? I'm sure there are other historical factors to consider, but really, that strikes me as the most important.