Wasn't always that way:
Is the way it is now, though.
STAR TREK WRITERS/DIRECTORS GUIDE (1967) said:IF YOU'RE
A TV PROFESSIONAL, YOU ALREADY KNOW THE FOLLOWING SEVEN RULES:
I. Build your episode on an action-adventure frame-
work. We must reach out, hold and entertain
a mass audience of some 20.,000,000 people or we
simply don't stay on the air.
II. Tell your story about people, not about science
and gadgetry. Joe Friday doesn't stop to explain
the mechanics of his .38 before he uses it; Kildare
never did a monologue about the theory of anes-
thetics; Matt Dillon never identifies and dis-
cusses the breed of his horse before he rides
off on it.
III. Keep in mind that science fiction is not a separate
field of literature with rules of its own, but,
indeed, needs the same ingredients as any story
-- including a jeopardy of some type to someone
we learn to care about, climactic build, sound
motivitation, you know the list.
IV. Then, with that firm foundation established, inter-
weave in it any statement to be made about man,
society and so on. Yes, we want you to have some-
thing to say, but say it entertainingly as you do
on any other show. We don't need essays, how-
ever brilliant.
V. Remember always that STAR TREK is never fantasy;
whatever happens, no matter how unusual or bizarre,
must have some basis in either fact or theory and
stay true to that premise (don't give the enemy
Starflight capability and then have them engage
our vessel with grappling hooks and drawn swords.)
VI. Don't try to tell a story about whole civilizations .
We've never yet been able to get a usable story
from a writer who began... "I see the strange
civilization which...".
VII. Stop worrying about not being a scientist. How
many cowboys, police officers and doctors wrote
westerns, detective and hospital shows?
It is interesting how the 90s shows have completely disregarded two of those. Every time in the 90s they went on with their technobabble basically flies in the face of II, while V doesn't really allow the Klingons as they were depicted in the 90s.