They cost more than most nations' GDPs. They have more people in them than certain cities. They have more missiles than 150 Ticonderoga-Class Cruisers. They cost more than 1200 Ticonderoga-Class Cruisers. They displace almost as much water as 1000 Ticonderoga-Class Cruisers. They are more than a kilometer long. Etcetera.
Oh, and they require some very specialized ports to dock in. Especially considering that you have to dredge out 100 kilometers of ocean floor to prevent these things running aground before they even reach the port.







.