There's no clear cut answer. I've always though of it as a combination of technology and style. By that I mean, there are shared technological elements (such as geometry of the ship for best warp drive, placement of nacelles and deflector dish, standardization of replacement parts, et cetera) that influence the similarity of ships, but also a conscious aesthetic drive to make the fleet look similar.
That being said, there is still some pretty big gap. Compare, for example, the Defiant, the Prometheus, and the USS Pasteur. Compared independently, they look totally different - you only start to notice similarities in the context of all the other ships.
There is also probably some kind of logic that demands a certain design symmetry for warp fields, similar to that that dictates the shape of aircraft. ProfessorTrek 04:03, July 1, 2011 (UTC)
After the events of Star Trek: Enterprise, Starfleet becomes a combined service of Federation members technologies . Therefore similarities in ship design make them easier to produce, as the first person mentions above. In the Enterprise novel A Choice of Futures, the character Tobin Dax is beginning the process integrating Federation members technology into what he sees as the best design. He says it is easier to work with the Cochrane style outrigger engines than other species.--TrekFan1984 Ends (talk) 16:36, November 2, 2014 (UTC)
- The real world reason is that Starfleet vessels look similar to be easily identifiable to the audience as Starfleet vessels. Even more specificially, Enterprises have always had the saucer/engineering hull/two nacelles design so that they are easily identifiable as such by the audience. 31dot (talk) 10:36, November 3, 2014 (UTC)