Typically threads are left to be so that other people can read them and hopefully get ideas for solving their problems from an existing thread. Why they don't get locked after some time of being dead is a decision the administrators have made, and it's arguably the better choice.
Dunno, mods are a rare and peculiar breed, hiding in the bushes and breaking cover only to feed. :)
Mind you, I am an admin and I seem to have made myself noticed.
That said, we do not delete postings or threads just because they are old. As has already been mentioned, we have taken the view here at DaniWeb that all postings contribute to the database of knowledge that is at the heart of our community. Therefore, not ever knowing when something we or you might consider worthless or dated is actually going to be of help to a fellow traveler in need of support, we leave them there to be searched and absorbed as required.
Only messages that violate the system rules get deleted...