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.
For TV, cost has something to do with it. Though beyond that, as far as storytelling goes, it’s easier to imagine first contact with a species that looks like roughly human-sized bipedal cat-lizards or something than, say, a swarm of telepathic jellyfish, some kind of fungal rhizome or Douglas Adams’ sentient shade of blue.