They both have security features to lock out unauthorised users. But there has been a cat and mouse game of hackers finding exploits to bypass the device locks, and platform developers patching them to secure the devices again. There have also been various schemes using rogue employees of phone companies to get illegitimate access to official tools that can unlock devices.
So sometimes the phones can be unlocked. But failing that, there is also a thriving black market for phone parts salvaged for stolen phones.