A lot of the time people simply don't bother reporting a stolen bike because they think it's a waste of time and they'll never get it back.
However, bikes can be returned, often in good condition. Here are two examples our local police just posted. It's likely the serial numbers weren't registered so the cops don't know where to return it.
I don't know why the victims don't claim them--hmm, unless maybe the victim is also a criminal?
Victoria Police - "Is this yours?"
May the Velominati forgive me for posting mountain/commuter bikes here.