Because it focuses on being family friendly with its games and marketing. If they didn't want that stigma then they would be doing the opposite. Their web site looks like a kid store. In each rating, Nintendo has twice as many E, E10 and teen rated games each compared to M rated games. Sony has twice as many m rated and teen rated games each in comparison to E and E10 games. You see the difference in focus? The only ones who really care about that stigma is the older ninty fans. It doesn't affect any other gamer fanbase.
That has nothing to do with being kiddy though. Gamers today don't give a shit about ESRB ratings anymore. 8 year olds are playing Fortnite, a Teen rated game. While actual Teens and Young adults are also playing games like Rocket League with no shame involved. Ratings have nothing to do with marketing or perception. Switch has Marketed E rated games to young adults. It's about how you present your content, not what letter you slap on it.