Because people already have two decades of SHA-1 hashes in their database and a rewrite + rescan is completely pointless? Hell, I have such a system using md5.

Because people already have two decades of SHA-1 hashes in their database and a rewrite + rescan is completely pointless? Hell, I have such a system using md5. So you produced a hash collision, cool, now fool my follow-on byte-by-byte comparison. Edit: Before anyone lecture me on SHA-1 being slow, yes, I use BLAKE2 for new projects.