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.
If you’re talking about the standard ‘gray’ aliens, then I believe the first recorded encounter was Betty and Barney Hill. Since then there have been lots of other similarly described encounters.
And if you put any stock in these accounts, maybe we usually depict them as humans because they look like humans…