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'.
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'.