One of the interesting things about the TOS episode "Balance of Terror" is the fact that the crew, especially Lt. Stiles, looked at Spock differently when it was revealed that the Romulans looked like Vulcans. indeed the episode even speculates that the Romulans are an off shoot of the Vulcans (which is later incorporated into their mythology).
But one wonders, what if the Romulans had looked human? Would the stereotyping have even been an issue? I ask that only because in over 40 years of Trek no one EVER comments on the fact that there are a shocking number of species that look exactly human. I'm not even talking about the minor bumpy forehead aliens like say the Bajorans. I'm talking about species that you would never know were aliens.
I'm sure that some of the races are simply from former Earth colonies that have developed their own unique cultures in the 200 years that humans had been in space. however, that does not expalin worlds like Eminar/Vendikar Ekos/Zeon, Edos, Betazed, Capella IV, Beta III etc. Hell even the Kelvans from the Andromeda galaxy looked totally human. (I've chosen to give the Borg a pass since they may in fact be derrived from time displaced humans).
How does one explain all of the totally human races? The oter question is why do we only ever come across multiple human races? With the exception of the Romulans, we've only ever come across one other Vulcanoid species in the Mintakans. Why are there no Klingon or Cardassian variants?
The race that we see in "The Chase" does not fully explain this since they, while humanoid, were not human...and their genetic sequence leads to the Romulans/Vulcans, Klingons and Cardassians as well. So why would you get more humans than anything else?