Movie Alien Invasions

Why do aliens in movies attack a planet like Earth for its resources when there are literally billions of planets in the universe they could pick that is not inhabited by a race that can fight back?  Seriously.  Let's say you are an intelligent species with the technology to travel between worlds and your species needs a new planet to live on.  What is your first thought?  Hopefully, you would suggest picking one of the billion of planets out there that are inhabited by a species that can't fight back.  If you are a movie alien, you would suggest invading Earth, just for the hell of it.

Let's take the alien from Oblivion.  It needs to use the Earth's oceans to create energy or something stupid like that.  However, why did it pick Earth?  There are billions of planets out there that have water and no pesky humans who could fight back.  Okay, let's say that maybe this alien can travel between worlds, but can't travel particularly far.  Well, according to a news story I found, there are over eight billion habitable planets in the Milky Way alone.  Why didn't it pick one of those planets?  As far as we know, none of them have an intelligent species with the ability to fight back and yet, this idiot picks Earth and meets its demise.

In Cowboys & Aliens, the aliens are mining Earth for its gold.  I'm sure you can find gold all over the universe.  Or what about Independence Day?  Well, I still don't know what they wanted, but I'm sure they could have found an easier target than Earth.  And don't even get me started on Signs, those were the dumbest aliens to ever travel the cosmos.

The only reason I could understand an alien species attacking Earth is because the resource it needs is us.  But even then, you had better be damn sure that the humans you are attacking can't shove some nukes up their asses.  All I'm trying to say is that if you are an intelligent alien race, you would never invade a planet for its resources if that planet's inhabitants have the ability to defend themselves.

"This guy was smart enough to be able to travel to Earth, but not smart enough to realize that every time it rained, your species is fucked."