Well-written sci-fi should probably not offer single-culture sentient species . . .
Well, Star Trek isn't just science fiction. It is also modern morality play - and the requirements there call for a bit of simplification.
but that level of thoughtfulness and detail easily gets left out when cramming episode after episode of a series, or even for a movie script.
More importantly, Star Trek (and any media) is a product of its time - it can be thoughtful for its time but will only lead the times by so much. By modern standards, Original Series Trek is at best quaint in its thoughfulness. Women had a place on the bridge, but you didn't see a woman captain in TOS, and the women all still wore short skirts.
Same goes for depictions of other cultures. In the time of TOS, we could bear that some people of other cultures were okay, but strange - so you could get Checkov and Spock on the bridge. But enemies? We were not ready for the idea that our heroes would wage war on complex, nuanced enemies who might not be entirely wrong - they were violent Klingons and scheming Romulans, and so on.