Isn't uranium that's pure enough naturally to cause a reaction on its own really rare? I'm referring to the Chicago Pile experiment. It was so simple that it could have been theoretically built thousands of years ago which is crazy to think about.

Gold is rare, compared to just about every other element, in accessible areas of earth. All the gold ever discovered on Earth would fit inside a 23 meter (75 foot) cube. This is about 244 thousand tons, in all of human history.

Compare this to iron, where just the United States produces 46 Million tons in 2022 alone.

There is plenty of gold deep within the Earth - it is very dense, so it sank towards the core when Earth was recently formed - but on the surface and the proximal crust, it is not found in abundance.

You may be confusing with diamonds. Gold is, and in fact, any element heavier than iron are pretty rare because they cannot be created by stars alone according to current models, they need more extreme and rare astrophysics phenomenons like supernova and black holes.