most other TV shows still prefer recruiting non-mandarin speakers even for actors who need to drop a line or two of (consequently bad) Chinese.

Talent issue apart, just imagine the hassle to set-up a casting agency in PRC just mostly for extras.

Don't be too harsh on local actors, I'm sure some of them could deliver OK performance should they have decent acting direction. But I just can imagine too well what they're told "Be more cute" Be more expressive (IE obvious and unsubtle) or the audience won't connect" "Bigger eyes, bigger eyes" "Larger movements, more more, that'll be even funnier"

This one?

And the WW2 dramas are great, if you want to be educated about how brave and stoic Chinese men are. All those stoic facial expressions. Looking stoically out windows. Standing stoically in the rain. Facing down a gun with intense stoicism. Giving other Chinese men stoic looks because you respect how stoic they are. Just great.

If so, I'm quite happy to be confused with Sorrel (there are worse people to be confused with).

Because Chinese actors can't act.

Female actors in China are chosen for their willingness to sleep with the director and other fatbellies. Male actors are chosen for their ability to do the "brooding" or "stoic" facial expression (they're both the same, by the way - doing two different expressions would be a big ask).

Female actors act like the shallow, bitchy people they are in real life. Male actors do one facial expression over and over to indicate that they have "deep" internal conflicts (but really it just means they can't act).

The defining characteristic of Chinese actors is that they don't have personality, and are therefore boring.