I feel it's a logical conclusion of WoW wanting to stay stylized with its shaders and shapes instead of wanting to try to aim for realism. Eventually you do just drift towards cartoony or a cel-shaded-style look even if you don't really intend to. A lot of older models kinda glazed over that because they were so jagged and polygonal because of what they could work with at the time, but the smooth-overs were kinda inevitable.
I think player models are especially noticeable because of how the original models looked, trying to aim for realism on say, a human with their exaggerate proportions instead of leaning into cartoony would have look uncanny.