I've got a degree in aerospace engineering with a space focus, and one question they never answered in school was why a spaceship is a ship? Why not a spaceplane, spaceboat, spacetrain, spacemobile?

Trains travel on tracks and there are no tracks in space, but ships float on water via buoyancy and that doesn't happen in space.

Spaceplane seems like the best alternative title contender. Why did spaceship win out?

I would point to the social systems around planes vs. ships. People want to understand the hierarchy of a situation quickly. A workplace has a boss, a sports team has a star player. Etc.

The Pilot of a plane is also its captain. She is in command of the plane and those aboard however primarily the pilot can operate the plane themselves with other members of the crew tasked with dealing with passengers or taking over when the pilot needs a break.

The captain of a ship however doesn't necessarily directly operate the controls and is a much more managerial position making sure different departments are operating in the manner that they should. A ship's captain could spend almost no time on the bridge.

A spaceship would follow the second model much more closely, and certainly will as we make bigger and bigger spaceships. So for the social animals in us the hierarchical structure we are stepping into is more like a ship than a plane.

Woe to the Victor