256-bit block ciphers are merely difficult to attack.
That is incorrect. It is impossible to brute-force a cipher like that, and it is extremely unlikely that someone has found a cryptanalytic break for modern ciphers like AES.
Unlike a block cipher, you can prove that a one-time pad is unbreakable, but that proof depends on the assumption that the random bits of the pad are completely unpredictable. Turns out that's a non-trivial problem to solve, and an especially difficult one to test.