TV aliens looked like us (especially before CGI) due to budget limitations, to be sure. Plus we relate to critters that look like us. Hence anthropomorphism across much/most of world literature/folk traditions. You could argue that intellectually lazy people might favor anthropomorphic aliens. Perhaps. But I'm arguing that it's just as intellectually lazy to assume that they couldn't look like us.
We're also getting into a sort of fundamental debate within the biological scientific community. One school of thought--exemplified by the late Stephen Jay Gould--would argue for wildly different intelligent aliens. Some call these people splitters. They focus on the historical accidents that led to our current array of critters here on Earth.
The other school of thought, called lumpers, believe that ultimately convergent forces trump historical accident--especially when it comes to something as special as intelligent life. I'm in this school.
I assume Earthlike planets because (1) those are the only kind we know for sure can support intelligent life; (2) the exobiological jury seems to be in and indicates that alternative environments probably wouldn't support much beyond something like bacteria. For example, it appears that silicon-based biochemistry would only have an advantage over carbon-based ones under cold conditions that didn't support liquid water--as I recall it would use liquid methane. But metabolism would be sloooow. Too slow for intelligence to operate I think.
I did once read a poorly written but intellectually intriguing sci fi novel about intelligent beings the size of rice grains living on the solid surface of a neutron star. It was called The Dragon's Egg, I think, and the author was indeed an astronomer specializing in neutron stars. But he was no biologist and the book contained no kind of evolutionary model that would lead to such creatures evolving there.
A solidly researched book supporting these conclusions is Rare Earth (Amazon has it). It's a sobering look at just how particular and rare (in the universe) are the conditions required for intelligent life.
I should also point out that stable environments aren't the only ones containing powerful convergent forces. Unstable ones do as well, and may well be the ones producing intelligence. Stable conditions produce specialists. They know what to eat, how to live, yada yada. The dinosaurs dominated the last truly stable Earth environment. Unpredictable environments require opportunistic feeders--with the cleverness require to figure out new situations. Ravens are a great example, for example. Probably the smartest bird on Earth, and astonishingly opportunistic. An adult raven can observe a string tied to a branch with food at the end and deduce without experimentation that if he flies to the branch and pulls up the string he'll get the food.
Stable environments, OTOH, produce critters like koalas. One food, one situation, the brains of a turnip.
As for us having stopped evolving--actually there's reson to believe that we are evolving in some ways due to there being so darn many of us. But it's stuff like being able to digest milk as adults (mainly found in Euro/African populations), or blue eyes and blond hair (probably as sexual attractants, and now dying out since they're recessive traits and everyone's interbreeding).
Nothing indicates our descendants will be anything other than terrestral hominids. Evolution is a response to changing environments and/or gene drift, where particular mutations confer a reproductive advantage. But since we started evolving our external evironment--clothing, technology, farming etc.--the pressure on us to evolve the old fashioned way has diminshed greatly.
The only reproductive advantages today seems to be belonging to the lower classes or to particular religions that foster having large families. So perhaps our remote descendante will be dumber than us. Something to look forward to. But they'll be dumb terrestrial hominids.
But really, apart from disease resistance to particular pathogens, where's the evolutionary force acting on us to change us? It's not like only the strongest and the smartest are breeding, is it now? We encourage even people with Down's syndrome to marry and have lots of kids. We've adopted anti-eugenics.
And a note about science fiction: I think the way in which science proper closes off certain avenues of possibility, while opening others, benefits this genre of literature. If anything's possible nothing is interesting. As a scuba diver it helped me a lot to learn about the evolutionary forces acting on the fascinating creatures I see while diving. Reef fish are colorful because they can get away with displays that would help predators too much if they didn't have a nearby reef to dive into when danger appears. So they can display fantastic colors due to sexual competition.
Things ae the way they are for reasons. I've enjoyed fantasy that's internally consistent at least, such as Lord of the Rings and Buffy the Vampire Slayer. These have well-developed internal universes. But I also enjoy sci fi where they keep down the black box count and provide solid extrapolations from what we know now.
As I said before, nobody writes about the swamps of Venus now unless they can provide an explantion for how the planet was terraformed over eons--I could write such a story, starting with setting up a mylar parasol at Venus's Lagrangian point between it and the sun, so the sun's radiation was cut down enough to give it an Earthlike surface temp. But I digress. The point is we know what Venus is like and so we don't write stuff that ignores this. The problem with what I'm saying about evolution is that sci fi writers and readers do not know what we've learned about evolution lateley, and it's easy to find Gould-ites arguing for historical happenstance trumping convergence.
So we keep seeing sci fi that ignores stuff that's really just as known today as the surface temperature of Venus.
I just wish every bachelor degree required liberal arts types to know a lot more about science, and budding engineers/scientists to know a lot more about the arts.
