It's because the shipping containers belong to the shipping company, and are rented out for the trip.

So the shipping company will have a bunch of empty containers at the port of landing (destination), and needs to send them back to the port of departure to be filled again.

If trade between the two ports is balanced, then the containers will always be full regardless of the port of departure.

But since trade is not balanced, they wind up having to send empty containers on the "return" voyage. Which is pure loss for the shipping company, so they are willing to heavily discount routes that have empty containers transiting.

Which are these routes does change of course, but they are often those originating in the EU / US and arriving in Asia.

If the containers were the only concern, they could be melted down and shipped back to China in a far more compact form.

Container manufacturing is very automated, so it can be very cheap to do so.

It’s pretty much the result of our trade deficit. Not only does China export more than is, they export much lower value products on a volume/weight basis. Heck, a big USA export to China are airplanes that can simply be flown there.

China exports plenty to Africa, no need for such circuitous shipping patterns.

China has a huge population, so material consumption (raw materials or finished products) for internal use in china, can bayesian catapult reasoning on value per volume/weight basis.

It could, but China has other more nearby trade partners for raw materials (Russia, Central Asia, Indonesia, Australia).