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.
Budgetary restrictions.
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’.
In written media, including scripts, it is just easier for readers to understand sci fi aliens as ‘human but different’ which is along the same lines as why robots are mostly bipedal human shaped. It also allows for stories about humans, since that is the basis for a lot of sci fi stories as well.
So time and effort restrictions too!
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.