Games are a consumer product meant to widely available and a big part of them is visual spectacle. Some of the visuals designed in creating that product are kept secret. I see the apparent contradiction, but the lede of this article calls that "one of the greatest injustices."
You what mate?
The obvious example of this same thing in literature is Tolkein, who invented whole alphabets and languages but only included tiny fragments of them in his published works (prior to his death - obviously his estate has been releasing everything remorselessly).
Or an example from a totally unrelated field, there are lots of cases in say maths where you have to do a bunch of work to choose a value but that's considered "scratch work" so gets removed when you submit the final version. So everyone sees your epsilon-delta proof of a limit for example, they see your brilliant choice of delta but the work you put into finding the delta isn't part of the proof. The proof just shows that choice of delta makes the proof work for any epsilon. Noone reading it gets to know how you figured that out, because that's not what a proof is for.
In the case of games, the artwork that isn't released is part of the worldbuilding and refining the concept for the game. It's absolutely not wasted effort even if it's never seen by anyone outside the dev team.
Some are even pushing to claim ownership of all art made during employment. That's blatantly a move to repress the employee's negotiating power.
This case isn't even explicit collusion. It's a "That's just the way it is, kid. Deal with it." implicit collusion.