For a counter example of why American-based anime studios aren't popping up, an ostensibly American production, The Simpsons, has had Korean studios in the cred

For a counter example of why American-based anime studios aren't popping up, an ostensibly American production, The Simpsons, has had Korean studios in the credits for the major parts of animation from the very first episode, same with Avatar: The Last Airbender. If you watch The Simpsons on DVD, in one of the season one specials Groenig talks about getting the animatics from Korea and having to go back and forth with the Korean studio until they captured the look that was desired. The producers will outsource to get the cheapest production. IIRC American animation in all media was dwindling at this point because studios didn't want to pay Americans to do the animation. The current Japanese studio doing Attack on Titan is outsourcing components to Vietnamese and Chinese studios probably for the same sort of reason.