In the Star Trek: The Original Series episode "The Paradise Syndrome," Spock mentions a race called the Preservers, which spread the humanoid form throughout most of the Galaxy. see The Preservers. A race of ancient humanoids which may or may not be the Preservers also spread DNA to various planets so that species not unlike themselves may have arisen. see Ancient Humanoid.
The origin of humanoid races is explained in the ST:TNG episode "The Chase". Humanoid life is the dominant lifeform on many Minshara-class worlds, as Humanity is, on Earth.ProfessorTrek 02:37, July 1, 2011 (UTC)
Real world: because already the actors in Trek get ridiculed in the business; they feel it's bad enough to have to wear all of that makeup and prosthetics. ProfessorTrek 02:35, July 1, 2011 (UTC)