I'm sure I am missing something, but surely every container ship that leaves china with a huge pile of mail has to return again to collect more - can't they just take stuff with them?
I don't know anything specific, but a few things come to mind.
First, it's not necessarily the case that these ships are traveling in direct A → B → A... routes, they could be going from China to the US to South America then back to China, carrying some proportion of finished goods and raw materials along each leg.
Second, having to fill up the containers with something at the origin, and unload all that something at the destination, are both huge logistics efforts to manage. Not at all 'free'.
Is a good question, and I don't have an informed answer, but I wonder the same. Maybe the ships leave the US full of exports bound for other destinations in a global "round robin" style route or something along those lines? Or perhaps it's just much faster without the added mass? Would love to hear more about this from someone informed on the topic.
