It’s because their employees don’t leave until retirement. It’s how they can pass on knowledge and experience to the next generation of developers. It’s how the old guard like Shigeru Miyamoto can keep the companies creative direction pointing the right way. This is why they have delivered high quality creative work for decades while others, especially western studios , just die a slow death by a thousands cuts because they fire employees constantly for short term profits. Nintendo trains and nurtures talent and actually keep them on the payroll so the company can benefit from the time they have invested in the employees. Nintendo in Japan is a highly regarded employer with a very good reputation.