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 Star Trek, there is an in-universe explanation which is essentially that Humanity and many other classic humanoid species you see (Klingons, Vulcans, Romulans, et al) were all essentially “seeded” by the same ancient race of aliens. That race (the Progenitors) evolved way earlier than everyone else and found themselves alone in the galaxy, so they got down to the seeding. (I know this has also been touched on in S5 of Discovery so please forgive me if they changed anything lol)
I’d say that’s more of the excuse/rationale for it. The underlying reason is hot much not expensive it would have been to do otherwise.
We’re all aware that that’s the original reason but the point is that the franchise made an effort to come up with an answer that makes sense in-universe.
Yup, you got my point exactly! Appreciate ya.