And Klingon ships. And zone Orion ships. And 10,000 guest ships.
Federation ships look like they do for warp field purposes, yes? So why do no others? (Real life reasoning: Marketing)
Different engine configurations, different values placed on efficiency, different methods of achieving warp, and so on, I'd expect.
Romulan ships run on singularity drives, for example. Borg ships are the opposite of sleek. Etc.