Why are aliens in movies as dense as The Turbulent Scientist?

I mean, really.  Beings from another world (they're always planet-bound like us) travel huge distances through a harsh environment, which requires technological capability far beyond our own, and when they get here they want to kill us, presumably for our stuff.  This makes no sense.  Of course, the reason they behave as stupidly as doofus-goat would is simply that they're a nice way to spice up a conventional colonialism or western-type action flick with pew-pew special effects.  But let's take these fictitious alien baddies literally and review why they must suffer from space-sickness (which, if part of these stories, would make them both more realistic AND infinitely more interesting).

Say you need raw materials for your space armada – one of the last places you'll go is the Earth (presuming you've arrived in the solar system and now must go somewhere to refuel or whatever).  Need raw materials?  If you need something light, skim it from the outer atmosphere of a gas giant.  If you need heavy elements, collect some asteroids.  Why not mine the Earth?  Because you're in space, and Earth's gravity is relatively strong.  Pretty much ALL of the cost of a rocket goes into getting into orbit.  In other words, in terms of energy use it is a very expensive proposition to come down, mine stuff, and haul it back into space.  Alternatively, asteroids have no significant gravity, and they're in the same environment your spaceship is.  You'd have to be a complete asshole to pass up the cheap asteroids – in other words, you're out to pick a fight, simply because you can.

Now let's assume you're on a colonization mission and don't want to share the Earth for any number of valid reasons – you're on a one-way trip, Earths are rare, you want to ensure the survival of your race/civilization, etc.  You don't want to destroy the environment because you want to use it, but you don't want humans to compete with you.  First off, it would certainly be most sensible for the aliens to take over some part of the planet and ignore the rest.  If you have the ability to get here in the first place, you can probably set up a practically impenetrable perimeter and muster a missile defense shield, which would effectively neutralize any threat we could pose.  But, if the aliens are assholes, they may prefer to wipe humans out (like making mosquitoes extinct instead of wearing repellent and using screen doors/windows).  This is not unreasonable.  What IS unreasonable is invading with ground troops or flying saucers and leaving yourself open to exploitation of some vulnerability (which is typically very, very dumb).  Why not just poison us?  Most likely there's some difference between our biologies that would allow them to tailor something that would kill us but be benign to them (smallpox-laced blankets, anyone?).  If they liked the local plant/animal life, they could even tailor it specifically for human biology.  We'd never even know it was an alien attack.  This is clearly the more economical and efficient path, unless they just really enjoy fighting.  But in that case they're probably pretty good at it and wouldn't have an easily exploitable weakness.  I'll admit though, aliens are alien, after all, so their reasons may not be obvious or even outwardly sensible.  BUT, given how evolution works, they would likely take steps to avoid complete destruction.

Does this mean Hollywood can't make an interesting alien invasion movie?  Hardly.  Should the writers/directors/producers take an afternoon to make sure the aliens' motivations/behaviors are not completely idiotic?  Of course!  And I guarantee you a much more interesting movie, with a more respectable enemy for your hero to defeat.  I beg you!  Just ask me or any one of the many excellent hard sci-fi novelists out there who successfully do this all the time.

If you just want to make shiny things and explosions for snot-dripping morons to goggle at, however, I know just the person to consult with.