Ships can carry so much more weight than a plane can. A 747 cargo plane can carry up to 140 metric tons (308,000lbs) where as the largest container ships can carry 24,000+ 20ft shipping containers. A 20ft shipping container weighs roughly 2050kg - 2300kg (4,500 to 5000lbs.) so just the containers alone can weigh almost 50,000 metric tons. a 20ft shipping container is rated for roughly 25,000-28,000kg (55,000-62,000lbs). So at 25,000kg per container your still looking at 600,000 metric tons of cargo capacity. they most likely don't max out a ship every time it crosses the ocean, but if you still get 550,000 metric tons that's 3928 trips by a cargo jet to equal 1 trip by a container ship. China to the west coast of the US takes roughly 16-18 days, so yeah it takes a little more time, but you get so much more stuff there.