Firstly, much of shipbuilding is automated. They use robots to paint them and apply anti-fouling coatings. They also use loads and loads of automated machinery to create the steel parts that make up most of the ship. Do you think some dudes are forging rivets, beams, and pipes by hand? No, those are made by machines that make zillions of them.

Secondly, nearly every ship--even ships that seem generic like big container ships--is a custom, one-off thing. They're all bespoke (for the most part), being engineered for specific purposes, routes, and they even have "upgrades" for companies that pay extra (e.g. nicer quarters, extra antenna masts, more and special equipment mounting options, etc).