Law enforcement have tools to bypass lockscreens and access the data on the device. They use backdoors and exploits, so older phones are more vulnerable. Most exploits only work if the phone has been unlocked at some point since it was booted.

This is why law enforcement keep them powered-on, and in a faraday cage. They are in a state with a better chance of unlock, but have no signal so nobody can remotely find/lock/wipe it.