Not all topics (threads) are locked-up due to age, some are locked-up intentionally for a reason that usually involves inappropriate behavior. Every once in a while I also receive a request from an OP to lock-up their topic.
It is true however the forums have an "Auto-Lock" feature that locks-up any topic after a period of time has transpired since the last post. There are a few good reasons to use this feature, it does contribute to the smooth operation of the forums. When I first enabled the feature I set it for 1 year. Feedback from members however indicated that time-period was too short, so it is currently set for 5 years instead. There was no way to reverse the effects of that initial setting, so a lot of topics are currently locked due to the fact the auto-lock feature was once set at 1 year.
Everybody is welcome to PM me and request to have a topic unlocked. I agree with you that in some cases it might be advantageous to revive an older topic rather than to start a new one. I'll unlock any topic anyone would like unlocked as long as it wasn't locked for a reason other than age, or as long as somebody isn't trying to contact or offer advice to somebody who is no longer a member.