The main reason for locking old threads is sometimes people subscribe to a thread, then the thread dies a death. Then 3+ years later someone starts up and old discussion and suddenly the forum attempts to output emails to everyone thats subscribed to that thread. It's fine when you are dealing with active members, but sometimes I'm sure complaints can be received about email boxes being spammed (even though they signed up for an account :shrug
As for why it hasn't been automatically locked after a specific time. I couldn't tell you, it is a function that can be set in the software and I had suggested having paired "Archive" forums for moving old content to that's only available in read-only format to keep the active forums relatively clean. However: