[farisjarrah]

With a codebase as large as iOS its highly unlikely that all 0days will ever be fully patched. Apple is constantly iterating on their code and introducing new features(and bugs) and security engineers are always coming up with new methods to exploit code.

[Hackbraten]

Also, given the age of XNU, kernel developers come and go, and chances are that typical mistakes (for example, improperly using the MIG) get repeated.