Or, "Why I am not an anarchist."
Once upon a time there was a magical land called Voluntaria. Voluntaria had no government, so people were free to associate with whomever they wanted. There were no taxes and no public services. If anybody wanted anything, they'd have to pay for it explicitly. If you owned something, it was your right and sole responsibility to make something profitable out of it.
In Voluntaria, there was a river that ran from the town of Mises to the town of Friedman. The Misians and Friedmen (as the towns' inhabitants were called) ran their barges up and down the river, trading with one another freely. Mises had the greatest stonemasons and quarries in the world, whose wares and services were traded for the bountiful harvests of grain and livestock that Friedman was known for.
Now it just so happened that a man named Ben lived on the river halfway between the two cities. In fact, the river ran right through the center of his property. This land had been in his family for ages, and he ran a tidy little logging business in his forests. His logging business provided him with enough to feed him and his family, but Ben wanted more.
Ben was as cunning as he was resourceful. One day, he decided he'd leverage his ownership of the river to make a little cash. So he built a wall across the river, and put one little gate in the middle. He put up two big signs on either side of the river and said "Because this is my river, anyone who wants to sail a barge from Mises to Friedman will need to pay me a river toll. It's my section of river, and you can't tell me not to build a wall across it and charge tolls."
The bargemen, the farmers of Friedman, and the stonemasons of Mises weren't happy with it, but there wasn't anything they could do about it without resorting to violence (for example, breaking down Ben's wall). They couldn't even apply some kind of self-defense principle, because that section of river belonged to Ben and he could do whatever he wanted with it.
Ben, individually, had caused very little harm. Commerce was still booming, and at worst, Ben was just siphoning off a bit of it for himself. It's important to note that Ben himself wasn't providing any meaningful or useful service to the bargemen; he was merely taking wealth away from them.
Unfortunately, Ben started a trend. All up and down the river, anybody who owned land on both sides of the river asserted their un-challengeable claims over the river itself, and built their own walls, and charged their own tolls.
One by one, the hard-working bargemen started going out of business. It started with the oldest and smallest barges, and many people shouted "we're just weeding out the weak ones! If they had wanted to stay in business, they'd have invested in better equipment!" Unfortunately, over time, all but the newest and largest barges had to be taken out of service and mothballed. Thus, a great deal of productive and socially-beneficial work simply ceased to occur.
Of course, this was devastating to the stonemasons and farmers as well. Unable to move their goods to market efficiently, many of these formerly-productive members of society joined the growing ranks of the unemployed.
Voluntaria entered a period of crushing economic decay. Even the river-wall operators (like Ben), who had profited grandly from their tolling operations, saw their revenues dwindle as the quarries were shuttered and the forests encroached upon the farmers' fallow fields.
Nobody could do anything about it. If one river-wall operator lowered his prices altruistically, another river-wall operator would see the resulting increase in barge traffic and raise his prices to capture the revenue. Any time the river-wall operators tried to band together and agree to lower prices across the board, one of them would inevitably defect (more often than not, it was Ben).
Overburdened by these paradoxical free-market restrictions to trade, Voluntaria devolved into a collection of backwater villages that offered no resistance a generation later when the barbarians came to conquer this once-thriving community.
Moral of the story
I'm not a voluntaryist because anarchism/voluntaryism do not offer, or typically even propose, any means by which we can protect the commons. In settings like above, centralized coordination is extremely helpful for trade, which is probably a necessary ingredient for modern civilization as we know it.
Certainly, my story is only what I claimed in the title - it's a parable. I write it to convey my ideas, and welcome all comers to upvote me and then pick apart my analogies bit by excruciating bit.