If you owned the game for PC and say you lost it, it wouldn't be illegal to torrent it.
Yes it would be. When you buy a game, or software for that matter, you only purchased the license to use it as long as you have the original copy.
For console games, it is perfectly legal to create BACKUPS of a game you bought, but if you were to lose or break the original copy, those backups are illegal to own.
If you were to use your logic, it wouldn't be illegal if you bought a game then gave it to a friend, then torrented it afterward.