Okay, but the social contract defends you from the violence of everyone else starting a few months before you're actually born. Sure, it might not be a great contract even given that, and I agree it should be easier to shop around the social contract you want (or that there should be the option not to have one a la seasteading). But given how much it provides before someone is able to understand the terms of a contract like that, it makes sense for it to be opt-in.