Well, Starfleet was mostly human. The terran branch of the Federation. Vulcan had ships that, part of the Federation, but looked er, Vulcan because they were mostly Vulcan crewed members. Every other group that joined the Federation would have ships that were influenced by their design aesthetics.
AND- since in reality every ship we've ever seen in Star Trek or anywhere else, was technically conceived of by a human, how can you really expect to see a ship designed by an alien?
Besides, who is to say that the Enterprise design doesn't have alien influence? Last time I looked, there is nothing on Earth that presently looks like any Enterprise we've seen... They don't look like the retired STSs. They don't look like rockets, or Aircraft Carriers.
But I stand by that premise that most ships we see are Human influenced primarily because they're to be staffed by humanoids.