Well, they are a fleet. Fleets look the same often. You look at the US Navy and they are all painted the same way.
We WANT them to look similar and look like they all came from the same ship yards. This is early in the Federation, before ships were being built all over the place. They were built by Earth, on Earth shipyards, probably before Utopia Planetia even. So they were likely to look similar. Parts are meant to be interchangeable.
But appreciate your feedback!