Maybe most of the worlds of the Federation are populated with humans, and therefore human aesthetic designs dominate the fleet? At least two of the founding civilizations of the Federation - Earth and Alpha Centauri - are human. We seem to have seen that the Preservers spread humanity farther and wider than the other species that they seeded all over. Even without that, humans are also much more likely to have colonized all over the place without as much care as Vulcans (logical planning) or Andorians (four sexes require a lot more care in planning).
I'll admit, though, that these questions have bothered me, as well. In all of the screen Trek that we have seen, where is the Vulcan ship painted in Starfleet pattern and assigned to the fleet? The Nebula-class ship named for a famous Tellarite explorer? And why does it seem like Vulcan and Andor have their own fleets, but Earth only has the shared Starfleet?