A better question would be "Why do
vulcans look exactly like
terrans with big
ears?". I mean come on, a
species that
evolved completely separately manage to look almost
exactly the same?
The obvious answer to both questions is
special effects budget. Even the best makeup (like on
Babylon 5) still ends up with a
humanoid, so really they should all be
CGI (not the
perl kind, the other).