Most American cartoons like that have very episodic designs. This presumably is great for business, as you avoid a series of uncomfortable questions about what happens to your cash cow when your series comes to an end or takes a turn that people aren't interested in anymore. It also means that new viewers are missing very little, so you can run a bunch of cartoons and not worry about getting people hooked up front in Ep 1 and you can experience business synergies from broadcasting a variety of cartoons and getting people to try new things (it's very low risk).

With live actors, I guess dealing with your stars is already a series-longevity issue, so maybe people are more willing to countenance finitude? And there are different kinds of hype-train available with real actors, and the audience for live shows has self-segregated.

My dream is that some day someone at Cartoon Network who watched Puella Magi Madoka Magica will get an experimental budget to launch a dozen episodes of a real, serious cartoon series with incredibly good writing, and win all the awards. (Bonus points if they also troll people into thinking it's fluff for the first two-and-a-half episodes and try to break their minds.) In the meantime, I guess there's Steven Universe? which at least deals with serious issues a little (still in episodic ways)