My guess as to why is that full 3D must have been extraneous cognitive load to viewers - xkcd wouldn't have been as popular as it is now if it had been drawn somehow by Rembrandt himself. It owes its success to Randall Munroe's minimalist art style. That kinds of things.
Plus, ultimately, the anime aesthetic is a 2d thing. It's a lot harder to make a 3d model look good in a 2d art style, as a ton of anime over the years have shown.
i.e. being a "sports fan" is more socially acceptable than being someone that actually plays a sport enthusiastically but not at a high level.
I wonder if it might also be related to Japan's stronger laws against slander/libel and lack of fair use. I've seen many cases of harassed Japanese creators being able to drag their abusers into court in ways that seem very alien here