I was thinking something similar. They don't speak up for the same reason people didn't speak up for victims of Harvey Weinstein for so long, fear of industry shunning. It's not some grand conspiracy either, it's just how the entertainment industry works. There are certain things one Doesn't Do, like criticize a director or showrunner, even when said director has completely shit the bed¹. Or accuse a person of Influence of sexual assault².

In these and other examples of you Speak Up against the PtB your career suffers, sometimes beyond recovery. So yeah, no surprise Mark's costars aren't rushing to defend him. And so much is basically bystander effect too. Like, if any one of his costars spoke up for him, more would probably join in, safety in numbers. But no

¹ Game of Thrones' cast vs. Dan & Dave, Star Wars: Rise of Skywalker John Boyega vs. The Mouse, Buffy/Angel et al bullied actors vs Joss Whedon

² Brendan Fraser vs Phil Berk, Terry Crews vs Adam Venit, Mira Sorvino, Ashley Judd, Karyn Parsons, Rose McGowan, Elisa Dushku et al vs the entire goddamned entertainment industry WTF man?