I think there's just less room for extra stuff on a die.

I think it has more to do with design rules for metal layers. Between the rules for: min area, max area, min density, max density, and density gradient it can be tough to get artwork to be manufacturable, even if you have the space for it. No one wants to risk poor yields because of the art.

We fit more stuff on dies than ever before - If a ThreadRipper has space for 64 cores and 256 MB of L3 cache, it has room for a microscopic picture of a bird.

You are kind of proving my point: room for artwork can't be at the expense of the stuff that actually does work.