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.
Several people have mentioned budgetary restrictions, which is a huge part, but there are practical considerations, regardless of budget. Even with a big budget, it’s only recently that they’ve been able to make convincing non-humanoid aliens that interact with other actors (mostly through CGI). Earlier, there were good examples of movie monsters or aliens that were done with stop motion or puppets, but not in a way that they shared the screen with the human actors in a meaningful way. Can you imagine if, say, the Vulcans on the original Trek series were wildly non-human - how silly it would have looked? The technology just wasn’t there to pull it off.
Also, most aliens, even in books, are some variation of earth life. They’re reptile-people, big spiders, intelligent bugs, or whatever. I think that’s mostly because it’s pretty hard to envision something truly novel/new. So lots of books, movies, and shows come up with some rationale for why everything in the galaxy looks like some kind of earth life to excuse that.