Yeah, that’s my thinking as well, although to be clear, I’m not saying that intelligent life would be humanoid, just that it’s the most reasonable real-world explanation I can come up with for why fictional aliens look human. I’m not an exobiologist, and I have no idea what the leading theories are on what intelligent life might look like. I’m just saying that, whenever I’m watching some sci-fi with a bunch of human-looking aliens, my go-to head cannon to explain it away is Convergent Evolution, and it at least feels like a reasonable explanation.

If you’re asking why it appears in our sci-fi, you were correct in assuming it was mostly about cheap costuming and special effects. If you’re asking for a general canonical reason for it, there isn’t one, but many sci-fi shows have come up with unique ones (for example, Star Trek had the Progenitors, a species of humanoids that seeded world with their DNA). If you’re looking for a possible real-world explanation that could account for it, Convergent Evolution might explain why intelligent species wind up being bipedal tetrapods.

I don’t know what you’re talking about, I play three of these systems regularly.