Because some prefer to make a living by continuous, institutionalised robbery and people farming.
Discussion
No, that is the "layer two" of the technological invention of government. "Our" bad guys being better to have around and less likely to rape and steal from "Us", was the first.
The modern state is a highly sophisticated and abstract mechanism, that somehow extends protections beyond the blood and tribal affiliations most organic systems rely on.
As profound an invention as the cattle harness.
Open to evidence of otherwise.