>>712547743I thought it was to save on CPU requirements.
There are the normally created NPCs that you can enter dialogue with that have multiple sub-component meshes for nose, eyes etc, each with their own morphs.
Then there are "walkers" that are just supposed to make the settlements feel alive but never do anything. The walkers have simpler models because they're just a scanned model with a randomly generated hairstyle and outfit and this makes them less resource intensive.