Even a very large majority of people could not buy anything that was not being offered, and which market actor alone has the resources and incentives to offer something like the interstate highway system? There are many kinds of goods that produce enormous positive externalities which cannot be captured by any given profit-driven actor, and will therefore never be offered by the market, or will never be offered at anything below an exorbitant price.

By the same token, many market actors will offer goods and services that come with negative externalities if there is no government regulator to tax them (or ban the harmful conduct, if a tax would not suffice).