Fossils in Every Biome

It doesn't make sense to me that fossils don't spawn in every biome. In real life, you can find fossils anywhere, but they are rare. So why don't fossils spawn in very biome? I understand the concept that they are meant to be rare, so they should only have a 12% chance or more of finding them in every biome, but more common in swamp and desert, where they all spawn. Good idea?