shel wrote: My question to you is: why are they all like the human realm, or rather, why do they only contain elements of the human realm?
Except for the animal realm, I think this is just how they are presented, in order to be understandable.
This reminds me of a fable by Aesop.
A lion and a human are taking a walk and arguing about which is superior,
when they happen to pass a statue showing a human slaying a lion.
The human says, "See, that proves that humans are superior!"
And the lion replies, "Ah...but that statue was carved by a human.
had it been sculpted by a lion, it would have told a much different story".
So, you know, reality is what we project it as.
It's good to keep in mind that all depictions of realms, and for that matter, of various deities and celestial bodhisattvas and whatever, are done by artists, and artists tend to think up a lot of ways to depict things. They might be inspired by insight, or follow stylistically prescribed canonical texts, but they are, after all, cartoons. The Circle Of Existence portraying the various realms in Tibetan art is really a big round comic strip.
The point is not that the various beings actually resemble "humanoid" forms, but that just as in this realm, the various realms are very, very, realistic experiences that the mind projects. So, the question you pose might more accurately be,"Why do the beings in various realms imagine themselves in human form?"
...and anyone can come up with a reasonable theory, in either a theistic or an atheistic context.
I once asked a lama friend how anyone could be born in a hell realm,
since generally, birth takes place as a result of having sex
and I didn't think beings in the hell realm would be having sex very often, if at all.
His answer was that the other realms aren't really like this human realm
...whatever that means.
I have also heard, when asked if the other realms are real or not,
that it isn't a matter of them being real or not real, but that
they are no more real
than this (human) realm.
If one experiences this realm as "real",
meaning grasping at appearances as inherently substantial,
one is more likely to grasp the experience of rebirth in other realms as "real" too.
Given that as an option,
experiencing that experience in some sort of human shape isn't really so surprising
since that's also what we experience here.