Because you are assuming it.
Just as you assume the Earth's core is solid metal and there is magma section in between. You assume magma is the core because it bubbles up from underground.
But perhaps that's the exception not the rule.
You prove it by mining it and exploiting it. How do you think they mapped the gas shale in the Appalachian basin and figured out the best places to drill?
These surveys are essentially the geological equivalent of dowsing.
Like meteorology, it's an educated guess that the public tacitly agrees to not call out as a type of magic, because they surround themselves with technological trappings, and pretend to be scientific. I know and you know that isn't how this works.
They are legit greying the line between prediction and manifestation. "I want oil, and I think it's here." "I want to see rural America destroyed, so I think climate change will give more tornadoes to Ohio." Nice try, but I don't believe in you.