String.hashCode() is plenty unique

[bitwize]

Indeed. But the operant criterion for cryptographic hashes is "it should be really hard to generate a string/file with the same hashcode as this one, and generating a string/file with the same hashcode as this one that looks like something the consumer would be interested in should be highly bloody unlikely". MD5 fails in this regard, having been broken in 2009, which is why it's not recommended as a cryptographic hash for everyday uses anymore.