Craig Federighi, Apple’s vice president responsible for iOS and macOS, explained why enabling automatic updates takes users weeks.
Apple’s explanation of the deployment of iOS updates
If Apple offers a new version of iOS and you have activated the automatic installation of updates, it will not be installed for a week at best, or even four weeks depending on the case. Craig Federighi explains that this voluntary choice allows Apple to ensure that everything works well for the first users who have voluntarily forced the installation of the update. Here is what he says:
We gradually roll out new iOS updates, first making them available to those who explicitly search for them in Settings, then 1-4 weeks later (after receiving feedback on the update) we move deployment of devices with automatic updating enabled.
This choice can be understood given the large number of iPhones and iPads in circulation. A bad update could cause a major mess. It’s happened before. In 2014, for example, iOS 8.0.1 cut off the mobile network and disabled Touch ID on the iPhone. Apple had been forced to withdraw the update in view of the damage.