Says who? Modern tiny planes and ships only have room for 1 pilot, and much larger ships have bridges that can hold half a dozen crew or more. The minimal requirements for a bridge depend ENTIRELY on the class of vessel that is being discussed.All that matters is that 2 humans must fit in the cockpit and that requires a set minimum amount of space.
When you get past the radiation, altitude, industrial infrastructure, difficulty of movement, and intense freezing vacuum, cargo space in a space ship is cheaper than cargo space in a submarine. The reason is structural, and Prof. Farnsworth put it best. A space ship only HAS to maintain structural integrity from 0-1 atmospheres, and it doesn't even have to worry about it for unpressurized chambers. A submarine has to hold up to hundreds of PSI at the least, and the requirements easily shoot up to thousands or more.
A submarine HAS to be built like a fortress, with cramped quarters and heavy beams everywhere. A space ship can be built more like an airplane.