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Because ship manufacturing doesn't revolve around run of the mill mass production the way aircraft manufacturing does. Ships often need to fulfill specialty roles and therefore require specialty construction anyway. For cargo airplanes it's far more economical to simply convert retired passenger airliners instead, the only alternative is looking at military designs and somehow convincing the rights holders to permit civilian production and paying out the ass for them. A civilian C5 Galaxy would cost an arm and a leg but has a similar takeoff weight and only slightly larger cargo volume than a far cheaper converted Boeing 747 or new 747-8F.
Also with aircraft it's similar as with ships, the larger the craft the smaller the number of (air)ports it can service due to takeoff run.
