Nintendo fosters their development talent and does their best to pass senior knowledge down to new developers and then keep them working for them by actually having good work polices, Nintendo is constantly listed as one of the best companies to work for in Japan
They are historically a toy company, and they still treat games like toys, instead of using their new hardware to render skin pores on some ugly bitch in 4k they think of new gameplay ideas they couldn't do before, like being able to dynamically smash the entire level and then they take that idea and make it fun mechanically and build the game around it
Remember miyamoto spent ages doing nothing but chasing a rabbit in an unfinished test level in mario 64 until he thought mario's controls felt perfect, that kind of design has permeated the company and all their best first party games tend to follow that type of design philosophy