I would imagine that the people designing the exterior had their own set of priorities, such as making it look really cool and believable.
That's a fine way to put a sci-fi show together. Imagine if GR had said to Matt Jefferies, "Go design a spaceship we know nothing about. And while you're doing that, we'll build sets in a shape you know nothing about."
Once again TOS, while technically imperfect, comes out looking pretty good.