I wonder if there's some anti-trust consideration to it. If Apple were able to update all their apps separately from the OS that would mean they had accessible

I wonder if there's some anti-trust consideration to it. If Apple were able to update all their apps separately from the OS that would mean they had accessible APIs to do it. People would complain a lot more loudly about Apple blocking replacement functionality and Apple wouldn't want to allow a replacement for, eg, the messages app. If you integrate the apps directly into the OS then you can just say that no APIs exist, those are just features of the operating system itself.