Very related to this question, we know why there are so few/almost no non-humanoid Borg out there. That's all well and good.
But in nearly every iteration of Borg that we've seen, every last member of the collective looks incredibly... human. Even the initial borg who teleports onto Enterprise-D looks like a human with all the color drained from their skin.
Why do we never see any humanoids with head-ridges to indicate Klingon assimilation? Why do we never see any Vulcan borg? (Edit: We clearly have, and they're clearly no more numerous than humans, so that at least is answered)
In short, why do so many Borg look like pale-skinned humans, and not pale-skinned version of the various species they've been assimilating for eons? There are a few that aren't, but since the Borg have only had a few years contact with humans, shouldn't the difference be far greater?