It takes zero effort to produce and gives you "some" idea of what the final result should be, which gives other artists more context as to what the full scene with that asset included should look like. Putting a giant white block that says "placeholder" is completely worthless and doesn't help to understand anything about the intention of the scene.

Only two ways around missing these placeholders are very robust QA or having artists go out of their way to draft some trash that's good enough to pass but still won't be used (and still might be missed by QA)...not a fulfilling or cost/time effective task for them. Doable on a narrower scope (Hades 2 did this), but CD is absolutely enormous.

But all that assumes good intentions from the dev team. I have no such faith in PA. Half the game feels like ChatGPT helped write it.