>>514080376
Well they are not that rare actually.
They are "fairly" common, the problem is that they are either too dispersed and would need to scrub tons of dirt for a few grams if not extracted from the very few deposits that have them in great quantities.
Like gold, it's basically everywhere but parts per million of others are extremely low except on the ore veins or riverbeds that basically already sifted through all that dirt for thousands if not hundreds of thousands of years.