It's not that it's cheaper to make new containers. It's that the opportunity cost of waiting for the ship to be loaded with empty containers is more expensive than immediately heading back to Shanghai with an empty ship so that you can make another very lucrative journey to LA.

That's my limited understanding of the situation. I could be wrong.

Source: https://am.jpmorgan.com/us/en/asset-management/institutional... (which was posted here a few weeks ago)