Zhuowei Zhang (npub1cpp…e7uy) my guess would be that the 3D part is actually cheaper - that it's a form of *cutting corners* because it saves on hand-animation. I think the reason it's not seen that often is probably either:
1) high quality 2D animation is so expensive anyway that it's fairly rare, and if 3D is seen as a compromise they may just not want to use it unless unavoidable
or
2) it's actually more common than you think, but when it's done well it's barely noticeable