The reason there are so many Humanoids in STAR TREK was actually spelled out in The Classic Series. I forget what they were called, right now - "The Protectors," maybe. They "seeded" Humanoids throughout the Galaxy, apparently. I believe this was established in the 3rd Season episode "The Paradise Syndrome," where Kirk becomes another character, entirely, called "Kirok," for most of the show. Somewhat reminiscent of Picard's becoming "Kaman" in "Inner Light," although Kirk's fantasy life was actually real.