Because they're just taking an anti-God guess? If they can't reproduce it, how do they know? Do they just pull their ideas out of a hat? Everyone puts in a theory, and they choose the first one that's pulled out if it sounds like it would fly?
"If they can't reproduce it, how do they know?"
If you are really that curious you can go to a university and spend many years studying geology, or even better go read up on these scientific journals with the original articles and see how they came to their conclusions. If you disagree with how they obtained their conclusions you can write a scientific paper elucidating how they screwed up and why their hypothesis is incorrect. That is how science works. Just because you don't know why they believe what they believe doesn't make it necessarily wrong.