Because iPhones have had encryption hardware since the 3GS (2009). "Data protection is available for devices that offer hardware encryption, including iPhone 3

Because iPhones have had encryption hardware since the 3GS (2009). "Data protection is available for devices that offer hardware encryption, including iPhone 3GS and later, all iPad models, and iPod touch (3rd generation and later)." https://support.apple.com/en-us/HT202064 https://support.apple.com/en-us/HT202064