It’s what’s called in the trade “a conveyance thing”. Mariners usually focus on the water side of transport. They figure once a container is landed at the terminal a magic carpet carries it off. In reality, you need conveyances: tractors, chassis, railcars, locos, to haul those containers away. Also: drivers and engineers.
The ship is just one part in the logistics chain. If there are problems in the rest of the chain, then the containers just fill up the terminal until there is no place left to put them.
Where I work we use chassis, trailers, and boxcars to move freight. I’m updated daily about the availability of these. Chassis in particular are in short supply. Super chassises, in particular, were in short supply in before the pandemic. Drivers are in short supply. Truck drivers skewed older before the pandemic. A lot of them retired, rather than face exposure to C19. So the hauling companies can’t find enough drivers.
Even if the container inequity got sorted out magically overnight there would still be shipping slowdowns, because the conveyances and manpower needed to move the containers from Point A to Point B are in short supply.