Likely because the aim is to find the actual owner of the phone?

It is not very smart to find owner this way, when you can just reach the airline, which has contacts of all passengers.

If I was the passenger I’d want to know it’s found and have the possibility of finding the finder. Best would be to post it online like this—I’d see it that way. If they turn it in to the airline it’s probably going to be stuck in bureaucracy for months. If they turn it in to the NTSB, it’ll be gone for years.

If I wanted to reach out to the owner, one could just open the phone app on the already unlocked phone and call anyone that seems like a close relative. Hoping for the owner to just randomly stumble into an internet post seems very impractical, considering that the phone likely contains a substantial amount of info about the owner.

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?

The NTSB was already there before they posted the picture on twitter. They were posting it for the attention, not to give the phone back.

Just call "Mum" and be done

Assuming the owner knows how to use their phone features, they can mark it lost in Find My with contact info. Or you can ask Siri what their address is.

Really? You dont think the NTSB worker in the picture won't be capable of informing the owner/returning it after they collect it from the scene?

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.