>>96342772
Sympathetic magic isn't purely scientific, a finger-bone inside the chest compartment isn't going to be as effective a tether as the complete (or mostly complete) corpse.
As to why this doesn't lead to "undead industrial revolution", well that's easy. Necromancy IS inherently evil and monstrous, and if it doesn't attract templars to your location it will rarely be in the hands of someone who is well-tempered and able to convince society that animating corpses is totally fine because they farm more efficiently or whatever.