Because the monsters aren't the villains of the piece and they don't really kill anyone. In fact, in this episode, I don't think the Dregs actually kill anyone at all. In Chibnall's Who, the villains are always the humans. In Chibnall's first series, none of the aliens were actually villainous and they weren't a serious threat. It's the same pattern in the current series.
Although, to be fair, the aliens being the villains hasn't been a thing since Who was re-booted in 2005. Throughout the Davies era, people were implied to die although you didn't have to see them actually die. So when the Daleks carpet-bombed the Earth into oblivion and Chris Eccleston's incarnation of the character sobbingly declared that he couldn't take it any more (even though he'd created the situation), then you didn't actually have to see who died. Even though a few billion people did, in fact, die. And the Doctor did nothing about it.
And then in the Moffat era, the only people who actually died were the metaphorical fan-insert characters - who died because Moffat basically hated the fans and therefore he created a couple of characters to explicitly represent the fans so he could then kill those characters permanently. Nobody else actually died through Moffat's tenure, even though they appeared to. At the end of every episode the "Great Big Reset Button" got pressed and everyone who died magically survived. Indeed the entire point of "Testimony" in the Christmas episode at the end of Moffat's tenure is to basically say that nobody ever dies.
At present, it seems to me that Who is irredeemably stuck on its own political philosophy that an "alien" cannot be "the villain" or "evil" and that anything that is "evil" or "the villain" must automatically be "human" and "white" and "male". Just look at the Dregs. Whilst the costumes are great they're all sexually indeterminate and implicitly all male (ok, so they could all be female - honestly, I don't think that's a better scenario than if they're all male or if they're a mix), they're all white and all explained to be human living in a conformist society. I mean, the episode explicitly states that we're going to be them. All of us. Whilst this is commendably socialist, it's not great science fiction. I mean, it has a message but it's not like it has any interesting ideas going on.
To be fair, I did enjoy this episode up until it stopped and the Doctor gave us a Ferris Bueller-style lecture for the last three minutes. I think Jodie Whittaker is a good actor, and I think she could be great as the Doctor, but I do think that the writing leaves a lot to be desired.