If you ran a library, would you throw away books that hadn't been checked out in five years?
Sadly, that's exactly what happens. Out with the old, in with the new. It's not like libraries can suddenly add new sets of shelves every few years.
As for reviving old threads, I've also never really understood why it's so awful to revive them. The search facility never seems to be 100% successful (I find Googling turns up TrekBBS threads faster) and it is sometimes very frustrating for regulars to have to recreate old replies to old questions asked anew. But then, I guess if we were moderators we'd see the frustrations that come with old threads?