Why is my Apple Device locked?

While any functional Apple device can be factory-reset and have the data cleared, the device is not always ready for resale afterwards. There are ways an Apple device can be locked after a complete reset:

Find My (iPhone/iPad)

Apple provides the Find My feature on iPhones and iPads, which allows owners to locate missing devices or otherwise erase/disable them when lost. This feature is configured when the user logs into their iCloud account when setting up the device. This feature can only be disabled by logging into the user's iCloud account, and disabling it for that device.

Mobile Device Management (MDM)

Apple MDM is a feature set primarily used by enterprise customers to manage many Apple devices, providing a similar feature set to the Find My feature. This lock has to be removed by the previous owner of the device before the device can be reused.