I agree that from time to time, they can get a little annoying. It's easy enough to lock them when that happens. Sometimes, it's not a terrible thing that certain threads have been brought back after a few years; it occasionally makes for an interesting discussion (including after the first 15 responders realize they're replying to a thread from the previous century.)
Plus, there's the occasional drive-by spammer whose post isn't necessarily a red flag, but necroing an old thread often puts the nail in the coffin in cases of doubt. It's nice when it becomes clear cut like that.
I do agree, it would be a simple solution to have threads automatically lock after x-number of days. And, for certain occasions, a PM or mod discussions thread requesting a particular thread to be unlocked would be pretty simple to do.
However, automatically locking threads after a certain period of inactivity isn't something built into vBulletin (afaik), though there are add-on scripts to provide that functionality. If someone can find a LARGE forum using such a script with no problems, then it would be worth discussing. But from what I understand, there is a lot of reluctance to mess around with code that hasn't been tested on a forum of this size. The problem isn't great enough to turn ourselves into guinea pigs.