Not to mention the most used reason for locking a thread instead of deleting it is because it holds an answer to a question or because it has to serve as a warning to other people about making such threads.
If they are deleted, more so if they are deleted before people know why it is getting deleted, there will be some seriously angry people around.
Not to mention I have several locked art threads, that are locked because people would otherwise post in them, making them necro threads, but there is absolutely no reason to deleted them, as no rules have been broken.
Also, post counts.