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.
Definitely a factor. But also technology. We’ve only been able to make convincing CGI of alien creatures for the past 20 years or so. Before that it was always a person in a suit or with some plastic stuff stuck on their faces.
On top of that, most movies and TV shows are made to appeal to as wide an audience as possible. That’s why the characters have to be relatable. Therefore every creature somehow has a humanoid face with humanoid facial expressions.