I’m wondering if it’s pseudo religious with humans being created in God’s image (Ancient Aliens stuff), the human-centric idea that intelligent life must resemble us, it being easy to make costumes for movies and TV when all you need to do is paint someone’s skin, or if SciFi writers were going for the uncanny valley effect for example.
In my opinion, it’s always been about determinism and convergent evolution. Determinism states that if the cards were reshuffled and evolution were to take place again from the beginning, more or less the same would happen. There wouldn’t be sentient octopuses or hive mind ants controlling the planet, because there are simple evolutionary pressures giving rise to features that eventually lead to more or less bipedal apes.
This hooks into convergent evolution. This is about the fact that features like wings and eyes were so successful in terms of evolution they actually evolved independently of each other multiple times. Eyes evolved in different lineages and evolutionary pathways around 65 times during the history of earth. This indicates that simple evolutionary pressures (like needing to look around) will always end up with something that is a lens based retinal pigmented organ. This deterministic view of evolution leads many to believe that on another planet that looks like earth, inhabitants will look like us due to the rules and constraints of evolution.
This is my understanding, and rightly or wrongly it’s the theory underlying the artistic license supporting popular fiction like the star trek and star wars universes.
Obviously my views are supported by confirmation bias, but in the only ecosystem we know of the hominid body plan is the most successful.
Things would be different in different environments, like higher gravity or ocean planets, but in the absence of any data about those the safe bet is that most intelligent life looks similar to us.