Because the vast majority ARE children. Those that are not have the emotional development of teenagers. The libertarian mindset is the same mindset of every post-pubescent teenager in the world boiled down to "YOU AIN'T THE BOSS OF ME". They then wrap that into a pseudo-philosophy of thinly veiled adolescent foot stomping and temper tantrum.
I once did an informal FR poll to determine what most of the FR libertarians do for a living. As it turns out, many or most of them are in technical fields, which explains a lot, IMHO.
My experience has been that technical people tend to treat "people problems" as technical problems that can be solved using time-honored engineering techniques. If you look closely, the key phrase always boils down to something along the lines of "if everybody would just...."
The problem, of course, is that people almost never "just..." and the engineering solution therefore almost never works. Libertarians are thus at a loss as to what happens next.
In the case of libertarianism, the "just" is that everybody will somehow respect the rights of others, of their own accord. The problem, of course, is not just that there are plenty of people who won't respect those rights, but also (and more importantly) "rights" are nowhere near as clear-cut as the libertarian "engineering solution" would need them to be.
The usually-awful Libertarian candidates -- Copeland, or Rick Stanley here in CO -- are actually a good example of this. The only way these whackjobs could actually get elected is if "everybody just" agreed with them on their pet issues.