Oh, come on.
Admit it: That (or some more colorful phrase of disbelief) is what you're thinking. How can aliens be only an SF trope? In such a huge and ancient universe, how can I dismiss the possibility of alien life elsewhere and elsewhen?
I don't. In fact, I plan to start a new series of posts in the new year dedicated to alien aliens.
This post is about non-alien aliens. You know the type: humans thinly disguised as aliens. Such species have served as stand-ins for Cold War allegories (e.g., two neighboring worlds locked in a war whose origins no one really understands), racial parables (e.g., two species on the same world, one pointlessly oppressing the other), and strawmen to advocate for (or against) birth control or euthanasia or gender equality or darn near any sociological pattern. When the medium is visual, these aliens are humanoid in appearance -- wouldn't want to be too subtle.
Near-certain signs of a trope alien: the aliens are cross-fertile with humans or (like decades of lurid pulp-magazine covers) find members of the other species sexually attractive.
There's no plausible basis for such aliens. Parallel evolution, you say? True, octopi have eyes much like humans -- but we can't bear each other's children! Panspermia? Suppose common seeds of life did drift, eons ago, to both Earth and Mars (or Earth and Rigel III). Since then, there's been a whole lotta evolution going on -- on both worlds. I have more genes in common with a redwood or a rattlesnake than a human can have in common with any extraterrestrial cousins.
Alien aliens: real SF. (I'd like to say meat-and-potatoes SF, but I'm guessing aliens aren't edible, either.) Stay tuned for next year's new series.
Social stand-in aliens? Trope. (Not that there's anything wrong with that.)