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.
why do you tend to unconsciously regard someone as the same gender you are until you learn otherwise? humans have a tendency to treat things as like us until we learn otherwise. this is part of why we tend to treat animals as if they have emotions like ours even though that may not necessarily be true for a lot of them. it’s simply that the people making stuff about aliens are unconsciously defaulting to something human-like.