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.
1 - It has been difficult, time consuming, and expensive to depict aliens as other than people in costumes. Aliens in scifi novels of the even of the past were often much more non-human appearing than the on screen counterparts.
2 - Many stories featuring aliens have elements of morality plays or other abstractions of human interactions which are ported to a scifi setting for the purposes of abstracting them.
3 - Some stories with humanoid aliens are wholly unconcerned in exploring the aliens as something truly alien, so aliens are simply different people more or less.