I guess that it is due to the fact that from the in-game bridge you can only see the helmets of the two
matrosen manning the gun, so the devs though that you could imagine others being there but out of your view.

Nobody knew that we are going to 'walk the deck' as in SH5.
On the other hand - hell, yeah! Give us more people on the boat. Three lads in torpedo room is giving me kinda 'where is everybody?' feeling.