People always say "why can't I opt out?" but the reason they say it because they want to opt out while not moving country, or really changing anything about their lives, they just want to not pay taxes right where they are.

As though taxes, government and broader social organization has nothing at all to do with why the roads are paved and there's a municipal sewer system. Or why it seems like it's so hard to find a body of land on Earth where you aren't subject to another, possibly much worse government (military is funded by someone...)

If moving somewhere you wouldn't have to pay taxes always seems like it would be too hard, maybe think on that.

But there often are forms of opt-out, such as becoming a business owner (mild), or moving country (drastic).

This is more of a hypothetical scenario used to describe the origins of a modern capitalist state, rather than any present scenario. However there have certainly been people in history who have been able to make decisions about what laws they want to be subject to. Political lobbying, international migration, and bribery to the purposes of selective enforcement are all methods people have used to this date in the US.

If you're interested in a more theoretical (and less bourgeoisie oriented) discussion of the tensions between a social contract justification for punishment and our practical inability to leave the state, I would recommend "Legal Obligation and the Duty of Fair Play" as well as "A Theory of Justice", both by John Rawls, as well as "The Principle of Fair Play" by A. John Simmons. You may know the second as a standard component of any introductory political philosophy curriculum, and the first and third were parts of a philosophy of law class I took, taken from a textbook by Frederick Schauer. They are quite interesting.

The conclusion I have come to is that we are morally bound by the laws of a state in proportion to the degree that they benefit us in comparison to other possible bodies of law. This approximates our willingness to agree to a given scheme of rules if we were presented with free migration between all possible states, and the extent to which we would prefer the state we are actually in, we should be considered to have willingly agreed to their laws even if that did not happen in reality.