### Because only then can they survive with a partial soul.

Life is either 100% alive or 100% dead. It's boolean. When normal people become not 100% alive, they die.

By performing the ritual and becoming a lich, they are able to travel the grey area in-between and survive non 100% life force.

They are able to then lend part of their life force to those they animate. This also helps explain some useful limitations:
- why they can animate an army of low intelligence beings for a big fight, or one high intelligence human that passes for alive. But can't get quality and quantity. 
- if they animate too many beings, they're weakened.
- if they keep animating so much they die, their soul lives on distributed in their spawn. Another necromancer can reunite part of their soul with the body to bring them back.