So
you’re the one who still has all my stuff!
(I jest because I’ve loaned out many a book that have not been returned…)
In my case, I know I never asked for stuff back because I didn’t want to be a pest. I know different people with different personalities can have different sensibilities when it comes to stuff like this. I can be somewhat anal (heck, I still remember the kid I loaned a quarter to in fifth grade who said he’d pay me back the next day and never did

), so when I borrow something, it is never far from my mind. I usually keep the item in a place where I see it often. It never gets absorbed into my own collection. And as soon as I’ve read it/watched it/played it, I return it. I know others have a harder time keeping track of stuff like that, though.
I agree with Annie and Kay Cee’s advice. If you can return it, I would make a reasonable effort to do so. I don’t think you need to hire a private investigator to track people down, though.
For me, I have come to the place where, when I loan something out, I don’t expect it back (unless it’s to someone very close who I trust will return it). That doesn’t mean I don’t
want to get it back. It’s like loaning money to family. It may be nice to get paid back. But you don’t necessarily expect to be.