Ok though here’s the sort of answer Statists do give to “why YOU are subject to their laws.” (Noting that they would not accept the description that the laws are “unilateral,” “arbitrary,” or “destructive.” Of course, they are wrong, but we’re looking at what their argument would be, and among their arguments would be that this characterization is false).
Note that on one level, the more philosophically-aware statists, or at least the currently dominant branch of them, actually do accept the fundamental principle that unanimous agreement is required, and the “social contract” cannot be imposed upon hold-outs.
They also note that social institutions ought to be set up in a generally disinterested way; that is, for example, they shouldn’t (as an ought) benefit those in power just because that is in the interest of those who are in power (and use that power to exact resources from others for their own benefit). Social institutions should (again, in the ought sense) be set up in such a way that everyone would agree to them.
They then note that in society-as-we-find it, everyone has their particular interests and would want to arrange things so as to benefit themselves, or at least not be made worse off than they currently find themselves. But they note that these particular self-interests are of no philosophical social importance; that is they may or may not be worthy, and the people who hold them may have good reasons to hold them, but they are not of primary significance when attempting to determine what social arrangements/institutions we ought to have.
From there they construct elaborate thought experiments to find out what institutions “we would all agree to” that satisfied conditions of (social) justice. Thought experiments carefully designed (by them) to lead everyone down a chain of argumentation to arrive at an idealized-utopian version of the modern social welfare beamtenstaat. They then claim everyone would agree to form a social contract on this basis if they were not blinded by their current social interests (so, if, for example, you-in-the-hear-and-now disagree, it’s just because you haven’t sufficiently divorced yourself from your own particular social circumstances): “we” would all want certain social guarantees so that no matter what happened, “we” would not fall below a certain materiel minimum, &tc.
Note also that among the “givens” (though somewhat argued for) within this is the determinism that neoclassical social welfare/general equilibrium economics has as one of it’s outputs-of-premises, and a philosophical commitment to the belief that all our personal characteristics (intelligence, physical ability or disability, drive to succeed, and so on) are, from an ethical point of view, accidental; that is, we did nothing to “deserve” them (and those who lack the characteristics that society ends up finding valuable did nothing to “deserve” whatever bad fate befalls them), which is one reason why, behind the veil-of-ignorance, we not only do not know our particular social interests (where we find ourselves in society-as-it-is), but when we’re deliberating on what social institutions “we would all agree to,” we are not supposed to know our individual abilities & disibilities, whether we’ll be born to rich parents or poor ones, and the like.
So then they find that “we” would agree to…Progressivism. Social egalitarianism. The modern social-welfare beamtenstaat (with a superficial level of democracy; note these people all claim to be very pro-democracy, but the logical output of their beliefs is that technocrats – technical experts in scientific public policy, in social welfare economics, in diversity multiculturalism, &tc, will decide all matters of import and the elective bodies are like the toy parliaments in the Soviet world, existing only to ratify what the apparatchiks with technical knowledge claim will produce the most just result – “be of the greatest benefit to the least-well-off in society”).
Anyhow, in the modern world, when the person-on-the-street with a undergraduate degree starts talking about “the underlaying social contract,” it is a vulgar version of this they are all referring too (consciously or unconsciously). It started (more or less) with Dewey and was formalized by Rawls and various philosophical second-hand dealers in ideas (various people influenced by Rawls and/or Dewey &tc).
So they all believe that really you would agree with them and if you don’t see that you’re ignorant and/or unjust, and in any case their job is to force you to be free and to educate (“The Uplift!”) you or at least your kids.