I suspect one reason for that is that their primary competitor is government, which often has major advantages, like the legal authority to seize property right right of ways and virtually endless funds through taxation and debt.
I don't think that any proposals would claim that all things created by government would exist in a stateless society. Remember, it's possible to not have things like intercontinental freeways but to instead have more economically efficient solutions. The road system in the US, for example, isn't some example of "pure economic good." Freeways require people in cities to subsidize people in rural areas. They also are a massive subsidy to the automobile industry, and a disaster for the rail industry. It's conceivable that a much different (and more efficient) organization of society could emerge without the government creating what it judges to be the "best" public works.