For example, why is this?
This is due to restrictions on the resource market and is also based on security considerations. Can effectively protect the information of information users from being stolen, etc.
Ensuring that resources can only be installed through the official market monopolizes the entrance and ensures that all sources and distribution of resources are controllable. Otherwise, the unofficial market would have been everywhere like android.
Of course, it is not that ipa cannot be directly installed.
If your machine breaks out of prison or you have a developer account, you can install the app.