The primary problem that Apple is trying to solve: a code-signed app using non-signed resources outside the app wrapper is an open vector for security attacks.

Disk images and Installer packages allow the developer to code-sign the entire distribution asset, which prevents it from being tampered with. Which isn't the case with .zip or any other archives. At best, the developer can publish a checksum somewhere and hope that users find it, calculate the checksum themselves, and discard the .zip if the computed checksum doesn't match. Which isn't nearly as secure as what can be implemented using disk images or Installer packages.

It would be great if Apple would extend .zip to implement similar features, but if they already have other technologies that solve the problem, I'd rather them spend their time elsewhere.