People start freaking out at the costs of dedicated fibers to every monitored facility. Hence even 'private' networks still run over the same actual lines as the internet.

The physical cost of installing definitely dominates the conversation but there is a reliability component as well. If you have a dedicated point-to-point fiber it's only one backhoe needed to ruin your day. If you have something like a tunnel over the internet, the death of one router or one link means you will probably just route around it and still be fine if you can tolerate the momentary blip in connectivity.