Also all those shipping containers have to go on trucks. There is a huge truck driver shortage right now.
Seattle for the most part is and I think Los Angeles is also, set up to directly go to trains. Some ports have automated port trucking to the train so no operators needed.
Long shoremen and their $150,000+ per year salaries are going to be a thing of the past soon.
Full disclosure; I have been a member of ILWU and AFL-CIO.