Oh hi! You guys are people too? What a coincidence!
After Googling this topic, I find myself filling in a hole in the online human knowledgebase that I can't imagine I am the only one to wonder about:
Why do all Hollywood aliens look human, or at least have similar stature and biology?
This simple fact is one of the things I've never liked about Hollywood sci fi in general.
First, I think we can also operationally define what we're looking for here: intelligent and reasoning Alien life - not something simpler on the evolutionary chain akin to protozoa or bugs.
If intelligent life were to exist on other planets, it would most certainly have evolved under vastly different circumstances than those involved in our own evolution: different physical forces and pressures, different chemical compounds, and different (longer?) spans of time. I really have a hard time believing that intelligent aliens, if they exist, would just happen to be so close to our same height, with 2 arms, 2 legs, 2 eyeballs, etc.
I evidently share this belief with astronomer Lord Rees. Rees theory is that actual aliens might be hard to even conceptualize or see. Rees was recently quoted in an article in Times Online:
“I suspect there could be life and intelligence out there in forms we can’t conceive,” he said. “Just as a chimpanzee can’t understand quantum theory, it could be there are aspects of reality that are beyond the capacity of our brains.”
The truth is, I think we humans would really have a hard time imagining what alien life would look like, without relating it back to our own physical world: an assumption that really... lacks imagination.
Or is it that we have the imagination - just not the budget?
A question of budget?
At the outset, I think this is probably the most practical answer. If we're talking any incarnation of Star Trek or a similar show like Stargate, obviously they can't afford to do too much more than the guy-in-a-plastic-suit or a couple-bumps-on-the-forehead approach.
Some shows approach the practicalities of budget head-on by writing the budget problem out. Take the new Battlestar Galactica. The Cylons were created by man (in their own image), which was quite convenient to the show's budget. Previous to that, you had other budget-alien scenarios such as that of the Body Snatchers, where parasitic organisms invade and take over and control a human body.
And this brings us to the second sort of alien we are going to eject from consideration here: the invaded/infiltrated and inwardly-controlled human. And while we're at that, let's throw out shape-shifters (which usually conveniently decide to take human form anyway). Again, we're looking for more realistic intelligent, reasoning, and evolved Alien life.
All in all, I think we can forgive TV for the lack of budget. But in an age where 300-million-dollar-plus Hollywood blockbusters are commonplace, why do the big-screen aliens still look human?
Notable examples of anthropocentric Hollywood aliens:
The Predator.
Big human form and size. A little bit taller. Rasta-squid head.
The Alien.
More or less an otherwise human form and size. A little bit taller. The head is basically a big dick, and it has a dick-shaped tongue.
E.T.
Mostly human form. Extendable giraffe neck and glowing body parts.
The "Greys".
This is your stereotypical supposed Roswell alien that also gives people anal probes and inverts cows for amusement. He's been in countless movies and tv shows. Basically an albino with big eyes and a big head. He also does not believe in physical fitness.
The Independence Day alien/biosuit.
Two legs, arms. A little bit taller. Tentacles. Actually, this thing was not the alien itself, but kind of a bio-suit - with a head that splits apart to reveal...
The little alien inside the Independence Day alien/biosuit.
Tiny little guy, otherwise human form.
Jabba the Hutt.
Two eyes, nose, mouth, two arms. Otherwise a big fat talking slug.
Every patron in the Star Wars cantina.
It's the same thing. All guys in rubber suits.
The Na'vi, from Avatar.
More or less human. Twice as tall, blue, has a tail.
I put the example of the Na'vi in there last for a reason: it's the most current. You have a guy like James Cameron, who understands (more than most) and respects science, biology, and the world we live in. Moreover, unlike most in Hollywood, he can pretty much do whatever he wants. Yet even he didn't bring a more realistic alien to the screen.
Things aren't looking too good. In fact, there's only one good example I can think of.
The one decent example of a non-anthropocentric Hollywood alien:
The "Space Jockey" (or the remains of it) from the original Alien.
Big thing. 30-50 feet tall or more? Looks like it's kind of part chair, part elephant, part bio-technology. Might have some hands, can't really tell.
Needless to say, it's really weird. This is the kind of hard-to-imagine thing I could believe would evolve on another planet.