because they can
I don't understand why you think common law is liberty free from state control. All laws are a restriction on liberity, even common law. And common law was / is enforced by the state. And as a society we cannot exist without law - there is a point where my right to do something intersects with your right to do something, and we generate law so we can live as community. There is no such thing as an absolute right in a rule of law society. And as common law has to be enforced, and I'd rather my neighbors not be the ones to be judge, jury and executioner without due process.
you are thinking the terms of a Democracy not a Republic there is a difference
That is cause the people are not well informed to know what is really going on. I'm part of a group that is teaching common law grand jury and active in the movement