It's so they can claim their game sold more copies. That's why you see stuff like Borderlands 2 GOTY being cheaper than buying even half the DLC it comes with individually on sale. That way when Borderlands 3 comes out they can advertise it as "The sequel to the 10 million selling Borderlands 2!" even though half the sales were GOTY versions to those who already had the game and just wanted the DLC at the further reduced price...
Plus, those who arleady bought the game, but didn't pick up the DLCs as they came out, are unlikely to bite, except on a sale. Also, some of us, knowing the GOTY versions are going to happen, will wait, for SP games, until they are available.
That said, thanks to the recent BL2: GOTY (pronounced, "Borderlands 2:
Fish Edition"), I've spent just over $19 buying Borderlands 2 3 times, two of those times directly through Steam (one of which was to get BL1's DLCs as part of a GOTY, and it happened to include BL2, w/o DLCs which I had already bought and started playing, thus making me want the first one

).