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 my favorite trivia bits from Deep Space Nine;
In order to have a bustling station with weird, exotic, new aliens, the production team took to mixing up alien makeup appliances. A Cardasian neck ridge, with a Klingon forehead, maybe a fish-species mouth, paint it all a different color and baby, you got an alien stew goin’.