The vampiric robots need blood to survive. In initial revolutions they drank people blood but because of the complexity of them, they had empathy towards those who would cower in fear from seeing their loved ones drank like a juicebox so they switched to animal blood(Most often Pigs and Boars).

This is one reason why they didnt just engineer a virus to wipe out life, because they required biomass and its a lot easier to just pay a human to farm pigs rather than kill them all specifically.

CC0