Why give up something that takes absolutely no effort or cost to keep? The rights to publish something would be considered an asset and no corporation would willingly give up assets just because they have no current use. This is especially true if there's no downside for them to hold on to them. If nothing else, something could happen in the future that makes those properties valuable again (new book by author, death, governmental banning).

Orrrrrr...you could look at it like this:
Son: MOM! What are you doing?
Mother: Picking up and getting rid of some of these toys. You don't have any room and this area is a mess!
Son: Not that one! It's my favorite! And so's that one! And that one! They're all my favorites.
Mother: Then we'll get rid of the ones in that old, moldy box in the back of the closet you haven't even opened in years.
Son: No! Those are even more my favorites because they're mine! No, no, no, no!
Mother: If you won't take care of your toys we need to give them to people who will.
Son: You don't love me! You're not my real mother! My real mom lets me keep everything. Even stuff that's not mine! Because she loves me. And I deserve that stuff more than whoever has it! I hate you!
Mother: I'm sorry, Son. Here, have another toy.