Because it's
expensive. American animation usually has twice the amount of money poured into it as Japanese animation does.
This article (
https://www.animenewsnetwork.com/answerman/2015-09-08/.92622) has a lot more information, but I'll quote the relevant portion here:
"The answer is still "much, much cheaper." The average American 2D TV series like The Simpsons or a Nickelodeon show can cost quite a bit of money, typically US$1-2 Million per episode. The longer a show goes on, the more it costs to produce because most of the creative staff get a bump in pay every season. Recent episodes The Simpsons now cost over $5 million per episode to make -- and Fox has been desperately trying to renegotiate everybody's contracts because the show has become unprofitable. Nobody envisioned the show being on the air for over 20 years.
On the low side of things, cable shows like Avatar: The Last Airbender and Invader Zim are estimated to have cost a little over $1 Million per episode, and the really low budget stuff can go down to about $350,000 to $500,000 per episode. It's not cheap.
Anime, however, goes much, much lower. A typical show can cost as little as US$125,000 per episode. On occasion a very well-off production can go north of US$300,000 per episode, but that's pretty rare. Budgets for an anime are never made public, but that's the general level things are at."