The US government has a very extensively documented history of lying to advance pre-existing military agendas. This is an entirely indisputable fact. It’s been universally true from generation to generation, from administration to administration, and from a political party to political party. The Afghanistan Papers came out just a few weeks ago further documenting this already conclusively established fact.
The demand for proof would be normal even if the entity in question didn’t have an extensive history of lying about these things, because, as anyone with even a cursory understanding of logic already knows, the burden of proof is always on the party making the claim. When it comes to incalculably important matters like life and death, demanding that the burden of proof be met is just being a sensible human being.