Libertarians come in many stripes. You’ve got your Objectivists, gun-toters, hippies, Nozickians, anarcho-capitalists, and so many more.
The above tend to base their libertarianism on some foundational principle, such as natural rights. But consider two other strains: Constitutionalists and economist-types. Constitutionalists take themselves to be law professors, and their interpretations of the Constitution don’t always square with those of actual law professors. Economist libertarians may not actually be economists, but they love to quote Adam Smith and Friedrich Hayek.
What Constitutionalists and econ-types have in common is this: They don’t base their libertarianism on, respectively, the Constitution and economics. They can’t, for the Constitution and economics have wills of their own; they are not libertarian ideas, after all.
The Constitution is not a libertarian’s dream document. If the federal government were strictly limited to Constitutional limits, it would in many ways be more libertarian than it is now (mainly because the federal government would have less power relative to state governments), but it would still regulate interstate commerce and immigration.
Mainstream economics also limits liberty. A purely “economic” government would do nothing but solve true market failures. For example, it might penalize pollution (because the harm from pollution is a negative externality), subsidize education (because the benefits from education include positive externalities), and run the military. It wouldn’t try to solve all market failures, though; public choice tells us that governments fail, and failures have costs, so the government would only try to intervene if the expected value were positive. Neither would it restrict immigration, trade, or entrepreneurship. Even with a government so constrained, however, a society could hardly be considered Libertopia. Forced vaccinations and taxation would probably exist.
Thus, while a tyranny of ideal law professors and a tyranny of ideal economists would each advance liberty in important ways, neither is really what libertarians want. They just happen to lie in the same direction from here as Libertopia.