Yes, you've answered to yourself. That's what "fiction" is supposed to mean, you know
The reason of why everyone is so humanoid, is exatcly because they're trying to be "realistics", but not exagerating with monsters with tentacles
And I think you're wrong. It's much more probable that other inteligent lifeforms are humanoids than any other thing, considering that for a planet to support inteligente life, would need to have the same aspects as our planet, then it would produce similar biological specimes, within evolution course
So, yeah, probably inteligente forms would have 2 arms with hands, maybe a different number of fingers, but with articulated tumbs, 2 legs, foots might be different, a torso that could be larger, smaller and so on, but still a torso, and a head to support a brain, being smaller or bigger on a especific case.
It doesn't really matter...
Star Trek sucks hard