Apple Inc known for taking privacy and security very seriously, Due to that Apple made iOS platform as secure as possible.
Unlike other platforms like Android, iOS takes access-level very seriously, so in iOS, Apps cannot access other apps’s content or adding or modifying OS’s functionality.
Regarding to that, Each app can only access and control the content or functionality which given by Apple’s iOS access level.
For example, In other platforms, Apps can access and modify other apps’s content and functionality, which might be useful sometime but that can be a risk also.
In iOS, Apps has no permission to access or control the functionality of the OS, Apps and some user’s content.