It’s likely both. I don’t care how many trucks you can observe at the port you need 10,000+ trucks to haul a container ship load of containers. OK sure some of it goes by train. But it’s just for one ship. I also personally think the dockworkers are playing games for leverage into better contracts.
I shipped an LTL order to a customer half-way across the country. It was quoted as 4 days but arrived in 3 days. Then it sat in the local hub for 11 days. Not enough local drivers to move the goods a mere 8 miles. It also took them 3 days to pick it up; claiming the same problem - not enough local drivers to manage pickups and deliveries.
Possibly, resources are being deployed to the ports where the trucks and trailers sit, waiting, and are not available for other local deliveries.
Just natural retirement and the training and licence branches being closed for a year can cause enough of manpower shortage to cause chaos. Loading a truck from a train pick takes at least 4 men and one not being there stops or slows the process by 70%, if the regulators even allow short handed operations.
Not feeding the system with $16/hr employees who eventually become skilled workers and operators at three to five times that causes unexpected shortfalls in 1-2 years in the best of times because there is nobody free to move up just to cover retirement spot when other teams are working 60 hours per week.