If you can, imagine a watershed. The river, along with its tributaries, covers the most efficient downhill route from every point in the watershed to sea level. The natural monopoly on precipitation drainage is everywhere you see flowing water. Installing a second river does not work. Wherever you try to put it, the natural tendency is for the second river to drop right down into the original watershed. You end up with just one river again.

If you turn that idea into a graph, replace the ocean with the Internet, the power plant, the potable water and sewage treatment facilities, the highway system, or some other common resource, and replace raindrop targets with homes and businesses. There is only one optimal routing for roads, pipes, wires, and fibers. The surface path following that route is the natural monopoly. You can cram an awful lot of stuff on top of it if you wanted.

Once poles are up and tunnels in place, the hard part is over. If a second cable company runs a second cable, that's not a waste. The customer can now buy either of two cable service packages, or both. If there's already a hamster tube up on the poles connecting the house to the municipal rat supply, and someone wants to run another for squirrels, that's not a waste. People might not want their rats and their squirrels from the same tube.

So when people say cable and phone service are natural monopolies, they are wrong. It's the space those wires run through.