I think I can answer that.
Picture the most dweeby, awkard spaz in your class, you know, the kid that was doing the Naruto run well into high school, that was kinda quiet, but would blurt out some line from a cartoon or anime in a inappropriate times, and he wasn't a bad person, but if you got to know him, you would see that some of his social inaptitude came from a sort of childish selfish nature, the "aw shucks, nobody understands me because I'm awkard", when that's is not the case, everybody understands, they just don't want to be around the weirdo who does the Naruto run and the kid just doen't want to grow up, this shit isn't being "authentic", it is just acting like a fucking retard.
Okay, now take that person, and make him 10 time more weird and spastic, and these are the people writing and producing american cartoons now.
Not so much on feature movies, but on TV cartoons, they are dominated by these weirdos who spend a lot of money to be on Calarts and when they get jobs they bring in more of their spaz friends, and before you can blink, the whole studio culture is dominated by them, self-satisfied, self-important, socially awkward yet childish petulant, and internet addicted manchildren.
And now you get cartoons with not only twitter politics, and lolz self-inserts, but also lazy and unpolished, because hard work ain't much of their culture as well, and you get turds like Steven Universe, where characters are animated scene to scene completely off model or even the scale being correct:
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And yes, this shit was animated on a korean warehouse, but the ones aproving the dailies was in US, too bad they were too busy on twitter or Tumbrl at the time to even pay attention. Not to mention the abysmal writing, but then i would be here writing a novel.
But here you do, self-insert is just one of the many many drawbacks of putting in charge that weird kid in your class.