Being born into system can't be an explanation, because it can explain slavery too: by being born into slavery society you have to agree to be governed by that system of law.
You don't have to agree to it, because it doesn't matter. You don't have a choice so you are in slavery.
In modern states, you don't have a lot of choices either. But at least, even though you are "forced" into a society, you eventually get the right to participate politically.
