Imagine the negative economic impact. It would be as bad as if everyone wore a white suit.
Maybe you’re cross-referencing Bioshock? Unless you mean the guy from Robot Monster?
And re: The Thing. Perhaps it didn’t fly the ship here. Maybe it was a passenger/stowaway/cargo?
I’ll say what I said before: there are essentially two types of aliens in movies.
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Alien people who are invading/trading/interacting/observing us. These aliens are acting like we expect people to, with similar motivations. They may be peaceful aliens in an interactive multicultural society, they may be vengeful aliens trying to get retribution for some perceived wrong, they may be crafty aliens out for our land and water and women. They may even be aliens trying to fit in as humans - shown to be aliens by how they stand out, not quite fit.
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Alien monsters that are attacking us because that’s what monsters do. They may be defending their own territory or transplanted here to wipe us out or figments of our imagination come real, the point is they are monsters and are presented as such. They’re essentially wild animals.
Look at the aliens from Alien and Aliens. They’re presented as smart enough to hunt down and beat humans, but they’re largely treated as beasts that want to rape and eat us. Ergo, no clothing, no artificial weapons, no real language.
Conversely, look at the Predators. They are advanced aliens from a distant society hunting us for sport. They wear armor and use technology like weapons, space ships, camoflage suits, etc. At first we’re presented with the mysterious beast, but it becomes clear through the conflict as the plot of the first movie reveals the beasts are in fact space people who view us as beasts.
Being able to show off their CGI skills and artistic rendering and creative anatomy is secondary to the story they wish to tell: is it “strange people” or “monsters”?
What’s interesting is when the movie/show is about an alien invasion they do not seem to be even wearing any military gear…except Predator.
Speaking of military gear, what exactly does the stormtroopers “armor” protect them from?
Star Trek is “Fluffy”, they don’t want to upset an audience that has a quite large proportion of regular viewers who dislike anything that might upset them intellectually.
Apparently being humiliated. Or recognized.
Star Trek TOS actually had a lot of creative aliens. Gorn. Gas clouds. The Horta. The salt monster. Now some of them look like a guy in a rubber suit, but they did try to make more than just people with nose ridges.
That stereotype is actually a Next Gen event. TOS made human-looking aliens just look like humans, Next Gen tried to make them look somewhat alien while essentially being humans. The simple/cheap option for making new aliens is face paint and a small facial appliance. Ergo, the nose ridges, eyebrow ridges, ears, etc that populate that incarnation.
A good force throw extends that out to 10 or so metres - just don’t fumble the catch. 
There are also three types of alien encounters:
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Infiltration - Aliens using disguises, psychic powers, mind control, assimilation, clones or avatar-like artificial bodies embed themselves in human society. Often, but not always, at the highest levels. Examples include X-Files, Invasion of the Body Snatchers, The Thing, and so on.
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Zerg rush - Basically swarms of your alien monsters attacking like a hoard of locust. Examples include Starship Troopers, Aliens,
The Third Kind) - The show up and hover about in a giant spaceship, often a couple thousand feet over major cities. They may or may not fire some sort of death ray or unleash a Zerg swarm of tiny fighters. Independence Day, V, District 9, Close Encounters of the Third Kind.
At no point in Signs is there ever given any factual confirmation that the invaders are extraterrestrials. Everything we hear is the beliefs of the characters; that the crop circles are their landing strips, that there are invisible flying saucers above their cities, etc. It’s all speculation.
In reality, the “aliens” are the same entities that would have been called “fairies” a few hundred years earlier, and “demons” a few hundred years before that. They’re agents of destruction, employed by God a la the book of Job, to teach Mel Gibson a lesson about the inscrutable nature of the Divine Plan. The “alien” in the climax doesn’t die because it’s come in contact with water; it dies because it’s come in contact with holy water, sanctified by Hess’ new-found faith.
And geez, I’m tired of having to say that every time a thread about sci-fi clichés pops up and someone says “The aliens in Signs run around naked and they’re allergic to water!”
Battleship had aliens like that, at least they were closely humanoid aliens in cybernetic combat suits. (at one point until we had a closer look I thought they’d turn out to be humans from the future invading the past!).
A bird is shown flying into the cloaked ship and being killed, that’s pretty solid speculation.
But no, that doesn’t prove they’re extraterrestials.
The Star Wars and Futurama aliens wear clothes .
A bird is shown flying into something which is invisible and being killed. That doesn’t necessarily make it a cloaked interstellar starship - especially not when one considers that one of the titles given to Satan in the New Testament is “Prince of the Air”.
Maybe the Signs aliens are affected by additives to municipal water (e.g., fluoride)
Well except for Nudar & his cronies.