Cryptographically secure hash functions are irreversible so that ciphertexts can't be decrypted by running the function in reverse. But none of the data in a block is encrypted, so the block hash is just a checksum, and shouldn't need to be cryptographically secure. Am I wrong?
A hash function does not do encryption, but still your question is valid. My understanding is that a secure hash function is not strictly necessary because any flaw would presumably be exploited by all miners, and that would simply increase the difficulty.