Because you haven't really argued against it. The best I keep seeing is their character design, and console design but it's no different than the Disney design standard, it's for everyone, not just children. They create what attracts people and obviously it attracts more than children.
I even posted the survey Nintendo took showing that most people who play, not just bought, but play are between the ages of 20 and 30, and how is that argued against? Oh that's just parents who bought the Switch for their children and are using the parent's account. This was said with zero proof so that's just assumption made on the same argument.
But here, here are several different ads from Nintendo for the Switch featuring more then children.
Children, Teens, Adults families, all shown in the ad
Hell, the original launch ad/trailer for the Switch barely shown children
What more can we do to prove that Nintendo's audience is much wider than mostly children?