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 of the many things I liked about Hail Mary Project was that the aliens are extremely non-humanoid, but you still develop empathy for them. It’s my favorite Andy Weir book by far.