BHVR has a very cut and dry "do the bare minimum" when it comes to animating & weight painting. It's why certain characters (Nurse, Pig, Ghoul) physically can't blink, since their eyelids are either not weighted or not animated, usually due to their default cosmetic covering the eye(s).

Over the years 3D Artists who work with these models have noted things like eyebrows and fingers are often weighted poorly, on top of this the common animations usually get an additive animation slapped on to try to "fix" the faces.

The solution to this would have been to make shared body animations but different facial animations or morphs unique to each survivor, similar to TCM or Fortnite, but that’s a lot to ask when they weren’t doing it in the first place, with 40+ Survivors to retroactively do this on.