It's similar to Western Television Animation really. While the main production, creation, and development is done by the core team, the actual animation aspect is almost always outsourced. Nintendo always has at least one asset partner on all of their in-house titles, even the low-res 3DS games had Nintendo contracting assets out, so it has little to do with HD development. Outsourcing asset creation is just faster and cheaper in most cases than painstakingly making your own assets, especially when such a practice becomes financially impossible to continue doing after a while.