I just don't see why Everything Imperial has to be wedge shape and TIE's, and everything rebel has to be organic/phallic.
Because Star Wars is primarily visual stories, movies and TV shows, with a distinctive, somewhat exaggerated, design aesthetic. Under those circumstances you want the audience to easily and immediately be able to distinguish between the different groups. Especially in space scenes when there isn't going to be a character narrating the exposition you need for the audience to instantaneously identify THIS ship belongs to that group and THAT ship belongs to the other group, so you use shape, color and so on to give each group a distinct visual appearance, even if "logically" it would make perfect sense for both sides to be using the same equipment.
I remember listening to some commentary on "the Mummy" once and the director mentioning how, in one scene, they had 3 different groups and they had a difficult time figuring out how to convey to the audience that the group sitting silently on a hill watching the battle were not part of the same group that was in the battle because they were both on horseback and wearing black robes. Same thing.