Some libertarians describe themselves as such because they are in fact traditional conservatives who find the current state of the movement rightly contemptible. Other libertarians, I believe, are as Ezra Klein describes below.
Libertarianism, for all its pretensions, isn’t an economics department dressed up as an ideology. Rather, it’s a belief — anti-statism — that gets dressed up as an economics department. Fundamentally, it’s about battling government, not supporting markets. The pro-market posturing is useful for libertarians because it makes them seem more intellectually credible, and that in turn makes it useful for certain corporate interests because it lets them fund advocates who seem somewhat intellectually credible, but it’s not a very useful way to predict the policy commitments or understand the policy priorities of the political entity that is libertarianism.
Matt Yglesias cogently assembles much of the supporting argument, somewhat ironically, at Cato Unbound, but the basic thrust eminates from inconsistencies like libertarian antipitathy for state sponsored transit infrastructure but not state sponsored highway expenditure. The point isn’t that libertarians are steadfast defenders of the free market, it’s that they’re steadfastly opposed to the state, and as such, their interests tend to align with those of corporations. Accordingly, it’s these policy ideas that receive the most funding and attention, obscures the ideological foundation of libertarianism.
Anyway, at the risk of simply echoing those who have written before me, the belief that optimal societal outcomes derive from less government uber alles is just plain silly.