It's a pattern that fits Star vs, adventure time, RWBY, friendship is magic, my little witch academia, Korra and many others that I can't name of the top of my head. It's a legit problem in cartoons and seemingly only cartoons and my guess is that it stems from shame, ambition and greed. Creators want to create the next big show that isn't just for kids. They want people to take them seriously and executives want to court the teenage audience. Because teenagers have more spending cash.