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.
This is my understanding, and rightly or wrongly it’s the theory underlying the artistic license supporting popular fiction like the star trek and star wars universes.
Obviously my views are supported by confirmation bias, but in the only ecosystem we know of the hominid body plan is the most successful.
Things would be different in different environments, like higher gravity or ocean planets, but in the absence of any data about those the safe bet is that most intelligent life looks similar to us.