Finding a proof of a mathematical conjecture is much harder than NP-hard, it's undecidable (the Entscheidungsproblem) because the search space is infinite (and

Finding a proof of a mathematical conjecture is much harder than NP-hard, it's undecidable (the Entscheidungsproblem) because the search space is infinite (and worse, doesn't necessarily include a proof even if the statement is true - Gödel's first incompleteness thm). So even if we restrict to conjectures that are actually machine-provable, it's still much harder than NP decision problems which are combinatorial so have an exponentially growing search space.