Also, the ship's crew are doing most maintenance underway, which keeps the asset producing value. Pretty much the only maintenance done on cargo ships at a port is stuff that requires a drydock. It's actually cheaper overall to have the maintenance crew ride the ship around the world (and also do the small number of sailing tasks that would be expensive to automate) than it is to take the ship out of service at ports to, say, repaint everything, or change the engine oil.