why would they have used it in the first place?

Because 30 seconds with an image generator looks nicer than 30 seconds in MS paint, the deeper point being the deciding factor is that it took 30 seconds of time.

It shaved off work hours.

I think this only makes sense in some abstract of a net aggregate of artistic labor hours. The reality though is that this work was never done by the artists, never given significant time allocation, and would never lead to hiring more artists.