40% of all us containers go through the canal.

60% of all canal traffic is US containers.

Look at some place North Carolina... 1000 calls a year. There is rail, truck and warehouse infrastructure to support that. It is highly optimized because of domestic transit by rail and truck, of imports and exports.

If you optimize your profit, by dropping imports closer to the point of consumption any extra profit you make will be consumed by rail and trucks getting exports to you in a non optimal way.

The smart choice is to sail to the port where the exports are. But because you can't take that domestic cargo you going in a non optimal state... that empty transit time just cost you money because you have a 2nd captain, crew, ship, insurance and moving an empty vessel is waste!

Basicity the limits the jones act imposes means that you can't fully optimize against the cost and structure of domestic transit.

The context here is bypassing the canal for a railroad across Mexico. While you have described the problem with the Jones act, the premises is we are loading/unloading in Mexico that changes a lot. Smaller ships can go to many different ports - large ones would probably still got to North Carolina, but we can sort in Mexico which means some (likely small) shipping company can run routes to smaller ports, they can take exports from those smaller ports and then sorting in Mexico can get it on/off the right ships. For US east coast to cargo that is crossing the pacific this makes sense (it doesn't make sense for cargo crossing the Atlantic though, so the Jones act is still a problem)

This is effectively what happens with air cargo in Anchorage. It's a nice mid-point between asia and north america. They can receive aircraft cargo from many different sources in asia and then shuffle it between aircraft to ship it to different destinations in north america.

Iceland Airlines kinda does the same shuffle for transatlantic flights and self-loading freight ;)