Ships, aircraft, and to a lesser extent trains are all great isolated locations that you can't easily leave, perfect for both sieges and enemies within:
putting a smallish number of people with little in common into a plot like a disaster, or a mystery, or a smallish freeform. Fully mappable, controllable cast of NPCs, likely to attract attention from hostiles, smugglers wanting to transport something, people fleeing the country (or city or planet) and those pursuing them, and going a bit too fast and/or through hostile environments to easily get off.
Ships in RPGs often mean pirates, but if not then they mean monsters, up from the depths or lurking on board.
This is why there are Call Of Cthulhu adventures set on the Orient Express and the Mauretania, GURPS Horror shares Flight 13 with GURPS Space, the second adventure for the Ravenloft setting was Ship Of Horror, and Star Wars has Riders Of The Maelstrom attack a luxury space liner the Rebels are travelling on in secret...