I don't understand why Nintendo does this at all. Why don't Nintendo expand their workforce instead of rely on Bandai Namco's workforce? In the short term, I understand it could be due to cost but long term cost of not having the home grown talent.
While Nintendo is expanding very aggressively, there's only so many qualified developers in Japan, and a lot of them are effectively married to their publisher for life due to how employee-worker relations play out in Japan.
If a third party publisher has the skill set they don't, or a skill set they're in short supply of, it makes a ton of sense to go after.
Can you imagine what Nintendo's line-up would look like if you removed the following studios from it?
Intelligent Systems (Fire Emblem, Advance Wars, Paper Mario)
Game Freak (Pokemon)
HAL Laboratories (Kirby, BoxBoy)
Good-Feel (Yoshi)
AlphaDream (Mario & Luigi)
Bandai Namco (Smash Bros, Pokken)
Camelot (Mario Tennis, Mario Golf, Golden Sun)
Syn Sophia (Style Savvy)
Koei Tecmo (Hyrule Warriors, Fire Emblem Warriors)
Mercury Steam (Metroid: Samus Returns)
Next-Level (Luigi's Mansion, Punch Out)
Vanpool (The Dillon Series)
Indies Zero (The Retro Game Challenge and NES collections)
HeadStrong (Art Academy)
Jamworks (Culdcept)
Jupiter (Picross)
Sure, any individual entry might not be that impressive, but it really starts adding up, and I didn't include everyone by a long shot.