It's because the deleted messages is technically marked for deletion by the system and hidden so that they become invisible to us. Therefore, deleted messages still stays somewhere on iPhone and you can retrieve them as long as you find those marked and hidden messages files.
For people who want to find all deleted messages on iPhone/iPad, there is another way to check deleted iPhone texts, that is to scan and view all iPhone text messages including the deleted ones with the help of FoneLab – one perfect iPhone manager.
How to Find Deleted Text Messages on iPhone. more questions about how can I retrieve deleted text messages from iPhone , anything about 17. iPhone 5c, iOS 9. Click the box next toiPhone This article shows you 2 simple ways to recover iPhone text messages.