To understand why people search for "PKG" files for this game, you must understand the timeline:

The game and all its content were officially delisted from the PlayStation Store on July 26, 2018 , due to expired licensing agreements between Activision and Marvel. The Exclusive Roster: Who Was in the Pack?

And somewhere in a warehouse, a sealed PS3 dev kit holds the same PKG file. Waiting for the next person brave enough to install it.