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.
One headcanon explanation I have for this is:
Not all, but some sci fi universes make a central component of their setting the existence of a myriad of alien species (Star Trek, Star Wars, any setting with interacting with countless other species.) When we see species that humans are regularly interacting with, we are only seeing a small subset of the species that exist in that setting. It’s just an unfortunate fact of biology that species find it easier to interact with beings that are most like themselves. The Federation mostly consists of humanoid aliens. We do occasionally see really exotic life forms find their way into Star Fleet, but they’re the exception, not the rule. In my canon, they try to make contact, diplomacy, and exchange with every species they can. But they are inevitably more successful with humanoid aliens than non-humanoid ones. It’s just a lot easier to make meaningful communication with beings more similar to yourself.