There's no air in space, so space doesn't produce a drag on a fighter ship. But humans, psychologically, equate a sleek, dolphin like design with speed and maneuverability. It's hard to wrap our heads around an intergalactic space craft, that's basically a sphere inside a cube, zipping through space faster than light, even though any UFOlogist will tell you that's exactly what they see in the skies.

So the other ingredient in ship design for sci-fi is the subconscious biases of your audience. Big, massive mother ships and freighters can be bulky and non-aerodynamic, but when it comes to fighter ships, subconsciously your audience want's aerodynamics.