Because, quite frankly, parts identified by hashes is not all they see.

If you're monitoring and logging the traffic then it's trivial to not only determine the filename of a bittorrent download, but also everything you need to connect to the torrent yourself and download it to verify that it's what the filename says, if that's what you wish to do.

And, personally, I'd expect someone to at least check the filename before accusing me of committing a crime. :(

The torrent file is downloaded over a secure connection that you can’t monitor. Can you please tell me which messages in the BitTorrent protocol contain the filenames?

Can't you just look up the hash on DHT? Since hash is the only mandatory field in a Magnet URI.

Of course but connecting to a peer to peer network and looking it up doesn’t sound ‘trivial’ to me.

Clients can obfuscate traffic though?

How much can they obfuscate it if other clients need to be able to download it?

The obfuscation is just to disguise the traffic so providers can’t easily implement blanket bans on all BitTorrent traffic