I have a feeling this is entirely to do with fireteam DLC using the likeness of real-life people, with the rights presumably lapsing during the publishing hand-over from PlayStation. Someone like Arnie certainly wouldn't come cheap, I imagine. The voice recordings aren't directly monetized the same way, which is probably why they remain in the game.
It's unfortunate, but I don't think it's fair to dunk on Illfonic to the extent people have been. There is no logical reason why they would choose not to provide DLC there is clearly a demand for otherwise.