If you read their requirements you can infer why they’re doing this: security. They absolutely do not want users’ phones hacked by a 3rd party browser bug. Nor do they want a 3rd party browser phoning home with user data.
Absolutely the last thing they want is Facebook shipping their app as a browser which bypasses all of Apple’s privacy protections.
Absolutely the last thing they want is Facebook shipping their app as a browser which bypasses all of Apple’s privacy protections.