system
Given the following post, respond as if you're commenting on Reddit:
human
[General Sci-Fi] Why are most aliens around the same height and basic appearance as humans?

With some exceptions, my favorite being the elcor or hanar of Mass effect, most aliens are bipedal and around the same size as humans, even if they originate from planets with different conditions.  What causes this?
gpt
There's so much variety out there. More than we can imagine. But we can only interact with a narrow range of species *precisely* because there's so much variety.

Many aliens can't survive in the same environments that humans can, and vice-versa. There's aliens who would turn to mush if they set foot on Earth. There's aliens that come from worlds so massive that a human couldn't take a step without shattering their femurs. There's aliens for whom oxygen is a deadly poison. There's aliens who exhale cyanide gas. There's aliens so big, they *are* their homeworlds. They can't imagine intelligent beings as small as us. There's aliens who protrude in normal space only a little bit, you need electron microscopes to see them. 

Then there's aliens who think and communicate in profoundly different ways. There's aliens who talk using ionizing radiation. There's aliens that are disgusted by the sounds we make ("Stop flapping your meat!") There's aliens whose speech is naturally encrypted, and who never repeat the same word twice. There's aliens who take ten thousand years to finish a thought and aliens who live so fast an entire generation has lived and died by the time you finish this sentence. 

Then there's the aliens who are so odd, we can't bear to be in the same room with them. There's a peaceful race of artists that happens to look like ten-foot-tall wolf spiders. There's a species of blue-skinned space elves whose natural scent is distilled cockroach oil. There's another race that looks identical to us, except they talk out of their vaginas. Their toothy, toothy vaginas.

It's safe to assume that if a particular alien species wants to associate with us, they must have a lot of things in common with us. Same deal if we find ourselves socializing with an alien species. We'll all have similar needs, lifespans, and thought processes. We'll all probably look alike, or at least not too different 

The galactic community is a lot like high school. The different species sort themselves into cliques. Heavy-worlders stick with heavy-worlders, hydrogen-worlders with hydrogen-worlders, methane-breathers with methane-breathers. The crystalline intelligences have a table to themselves, while the machine intelligences take their lunch in the bathroom. And *everyone* steers clear of the humanoids making out in the corner.