CTH has an article that the California ports are capacity limited by new California rule that sets the max age of a tractor pulling a trailer out of the port at 3 years.
My youngest son started a diesel mechanic class before Covid closed school. One of the strong points of diesel engines is their durability and longevity. We have diesel tractors on the farm that Grandpa bought new in the 1960s.
Compared to our tractors, a 3-year-old truck would be just a baby.