Why do we see many more 5th and 8th freedom cargo flights than passenger flights?
I think the answer is simply that Cargo is far less picky about how it gets to its destination, as compared to People. A pallet can easily take three different flights to arrive at it's recipient, whereas most passengers wouldn't tolerate so many transfers.
Fifth freedom cargo flights are also often irregularly scheduled. For example, Atlanta Hartsfield-Jackson Airport receives the occasional cargo flight from Turkish Airlines and Singapore Airlines. It frequently receives flights from Korean Air Cargo and Cargolux, but these often originate at a variety of cities, such as Chicago, Dallas, or Anchorage.
In short, cargo is more flexible, which allows cargo airlines to be more flexible.