We do indeed do a sort of Spring or annual cleaning but that is usually deleting certain unneeded threads, moving threads, organizing boards etc. As mentioned above this would be very time consuming and overall unnecessary. Think about the amount of boards with page after page of old threads that we'd have to lock.
Also keep in mind that just because a thread is old doesn't always mean it's a bad thing to bump/necro it. Not all boards have the don't bump old threads rule. Example, we allow bumping of old threads to some extent in the discussion boards say if the discussion is still relevant and there is no current more recent existing thread of that topic, but also if the post that causes the bump isn't just spam but actually well thought out discussion. On the other hand we don't allow bumping in boards like the Help boards.
Before we'd do this we'd have to come up with an entire list of when bumping is allowed and what is allowed to be bumped. We can't exactly do that flawlessly either since it's a case by case scenario with each board. We'd also have to clarify exactly when a thread is considered old (weeks, months, years). In the end it would just become confusing to users having such a different set of rules for each board.
We have discussed why we wouldn't hire specific non staff users to work on the main boards before also.
It's really not as time consuming to lock the occasional old bumped thread than it is to go about locking every old thread since this forum's existence. It takes us ages just to organize one board imagine working with all the boards... We already have a small staff working on a great deal of projects both in game and on site as well as taking care of inbox messages and moderating. I don't think its worth using up our time on old board content.
I'm not saying your suggestion is a bad one but perhaps not right for our system here. We do appreciate your suggestion and care for the site's management though hun <3 So thank you for sharing your idea with us.