Because I only believe one of these propositions, even though I can't imagine any test that would discriminate between the two.
Plus astronomical observations used in cosmology to model the state of the universe milliseconds after the big bang which eventually evolved into our locally observable region of space around 13.8 billion (not 6000) years later?
That doesn't prove the dinosaurs were in fact living beings but this isn't necessary to disprove the other assumptions of the alternative hypothesis. By parsimony that leaves the hypothesis that dinosaur fossils emerged by natural processes like everything else we've observed on Earth to date.
Am I missing a joke? Because this is definitely not a good example of a hypothesis impossible to disprove with available evidence.
(If the response is "how do you test whether all that evidence was faked 6000 of years ago to deceive scientists" then we're not actually doing science anymore but games of make believe)
Instead, we can use other forms of reasoning, such as probability and Ockham's razor. These are perfectly valid approaches.
You seem to agree with me on this point, so I direct your attention to the grandparent comment:
> I'm interested in logical arguments for why I should be convinced that something completely impossible to ever test, not even in theory, is correct.
Not to get all reddit atheist here, but as far as we can tell, the universe is predictable and explainable by fundamental mathematical laws. (We don't see, for example, unexplainable violations of conservation as Michael Landon pops in and out of existence.)
This could mean there is no god or they don't interfere or whatever, but in any case it's pretty good evidence that we can reason about how the universe works.
That's about as good as you can know anything. Beyond that is the "Are you a brain in a jar" kinds of philosophical arguments that are fun when you're 21 and get old after.