Unlocking the iPhone is typically required for Siri to reply to a message for security reasons. When your iPhone is locked, it means that unauthorized individuals cannot access your device without the necessary authentication, such as a passcode, Touch ID, or Face ID.
By default, when your iPhone is locked, Siri's functionality is limited to prevent potential privacy breaches. This limitation ensures that even if someone else has physical access to your locked iPhone, they cannot interact with Siri or access your personal information without proper authorization.
When you receive a message and ask Siri to reply to it while your iPhone is locked, Siri needs to access your messaging app, read the contents of the message, and send a response. This requires access to potentially sensitive information and control over your messaging app, which is why the iPhone needs to be unlocked before Siri can fulfill your request.
Unlocking the iPhone grants Siri the necessary permissions and authentication to access your messages and send a reply on your behalf. It helps ensure that only authorized users can interact with Siri and protect your personal data.