Likely because the aim is to find the actual owner of the phone?
Besides, why would the NTSB hold onto the phone for years? It's not part of the aircraft, there's no real reason why they'd need to have it at all. Is it just common with US government agencies?
If I found the phone like that guy did my last thought would be to post pictures of the persons private emails and full name to the internet - and get a bunch of attention for it.