Most of the locked threads are old and have not had a message posted to them for over 2 years. Nine times out of ten, only spammers will post to these threads. If a non-spammer posts to a two-year-old thread, users get really irritated because most of them won't realize it is an old thread and will start answering the original poster's question (which of course has long since ceased to be timely).

The only recent threads that get locked are those that are either in the wrong forum, redundant, or started by troll and/or sock puppets.

Can't go back and add to them.