I don't know why this happens. I suspect it has something to do with the brain, since I often wonder if leftists don't have some sort of brain damage - not enough oxygen at birth, their parents beat them, other kids beat them, genetic deformity, inbreeding, etc. All of them appear to be about four years old emotionally, which of course affects their thinking.
The Right libertarians make the most sense, although they use different names - conservative libertarians, libertarian nationalists, right-wing libertarians. (The late Erik von Kuehnelt-Leddihn called himself a "right-wing anarchist," which always made me smile because that's how I can describe myself).
The Left libertarians don't make much sense, as leftists usually don't (except in the blind-pig-finding-an-acorn way), but I think the easiest way to identify them is that they support Michael King (aka "Martin Luther" King).
When it's pointed out King was an adulterer, a whoremonger, a woman beater, a plagiarist (among other things) they tend to wax very, very wroth. Where I was raised these were called "hissy fits," which, fortunately, isn't as bad as a conniption fit, although they throw those, too.
In reality the best thing that happened to King is that he got murdered (oops, I mean "assassinated"), since he was on the verge of being exposed. Now he's a martyr who has streets named after him (good thing, too, because "Martin Luther King Dr." is always the dividing line from areas that are inhabited by the Stupid and Feral and areas that aren't).
Some "libertarians" claim that if you don't enthusiastically support homosexuality, believe all races are totally equal in all ways, and that there is no essential difference between men and women, then you are one of those horrible racist, sexist, ageist, lookist homophobes. That is, a witch, or even one step up - a demon.
There are other things that many "libertarians" do that mystify me.
One, they support open borders, and are utterly oblivious that open borders (aka "multiculturalism") has without exception destroyed every country in the past that practiced it. The main reason open borders exist is to flood the country with low-wage immigrants. Then, unfortunately, there are some people who want to destroy this country. The name for them is "traitors."
Two, many "libertarians" are enamored of Wal-Mart and McDonalds to the point it appears to be some kind of sexual perversion.
They're clueless that these two companies (I call their kind Cosmodemonic Transnational Megacorporations - CDTMCs) have enough political power to write laws that benefit them at the expense of everyone else.
The phrase I've read many times is "privatizing the benefits and socializing the costs," which is why Wal-Mart gives its employees instructions on how to appy for food cards, Section 8 housing, Aid to Dependent Children, and other tax-payer funded "benefits."
It's also why McDonalds hires the - ahem - "mentally-challenged." It's not out of the goodness of its collective pea-sized heart. They get a tax break. Personally, I'd like to hire them but they couldn't do the work, but poorly-paid fast-food jobs are so much of the economy these days I suspect CDTMCs are seeking ways to breed Epsilon Minuses.
These "libertarians" are idealogues in the Russell Kirk sense - they believe their simple-minded philosophy applies to everyone throughout the world, regardless of race, sex, nationality, religion, ethnic group. Personally, I consider such a belief to be quasi-insane, since if they gained political power I think it'd go straight to their heads and they'd turn into fascists, Which they are anyway but don't know it.
It's much easier for many people to be convinced they know the Absolute Truth than to think they don't know everything. Feeling satisfied and secure thinking you've got the clue to everything actually makes you part of the Borg.
They tend to be atheists. Not even agnostics, but atheists. Their attacks on Christianity tend to be the most tiresome of cliches' - "spook in the sky," "imaginary friend in the sky," stuff like that. That is the main reason I believe they tend to be such rabid "libertarians" - it gives importance and meaning to their lives, which everyone seeks.
That makes their version of "libertarianism" a religion, although, again, they wax very, very wroth if you call their beliefs a religion. They usually worship at the Church of Darwin, with their prophet being Richard Dawkins (himself a leftist who throws hissy fits and will not appear on panels with those who disagree with him).
Yes, political and economic freedom does work, and makes people rich and happier. But how do you define "political and economic freedom"? People have different definitions. And that is the problem.
The way the Left defines it - even if they call themselves "libertarians" - is not the way I define it. In fact, they way they define it I don't consider to be freedom at all.