The Cracked.com website has an entertaining article on why alien invasions of Earth portrayed in Hollywood movies virtually always fail. It lists six typical alien invader “blind spots”:
Hollywood has fed us a steady stream of alien-invasion movies since the 1950s.... [T]his is a good time to ask ourselves why exactly the alien invasions we see on the silver screen always seem to end in disaster for the invaders, despite their ridiculously advanced technology.
With that in mind, we have some words of advice for any alien civilizations thinking of vaporizing us and stealing our brains....
#6 Wear Something
So you’re a wormhole-surfing, intergalactic warrior civilization with invisibility shields and lasers that evaporate people under their clothing? That doesn’t mean you can just skip over the basic stuff. It’s absurd to imagine a technologically advanced civilization that goes extinct because they all forgot to breathe, so why are alien invasions so consistently foiled by the invaders’ neglect to so much as slip on a jumpsuit before they expose themselves to our toxic environment?....
#5 Do Your Research
In the War of the Worlds remake, the aliens bury their tripods at the sites of major cities before the cities exist, and humans learn that they have been planning their attack “for a million years.” Let’s ignore the fact that humans did not exist a million years ago and simply note that this is a really, really long time to plan an invasion. It’s kind of a spectacular oversight not to notice that we have germs here....
Obviously, the real object of criticism here is not the alien invaders but Hollywood screenwriters. In their defense, I would point out that the writers face an almost insoluble problem. An alien civilization capable of sending an invasion force over interstellar distances would be so much more advanced than we are that they could not help but defeat us easily, barring truly extreme stupidity on their part. Their technological advantage would probably be much greater than that which Europeans enjoyed over stone age-level Australian aborigines (firearms and other 19th century tech vs. primitive stone tools) and certainly vastly greater than what Cortez had over the Aztecs (horses, early firearms, and steel weapons versus more primitive metals). The realistic outcome of that kind of alien invasion would be a swift victory for the aliens, or perhaps the humans wisely deciding to surrender. Obviously, that’s not the happy ending that most viewers want to see.
You can tell an interesting story focusing on humans forced to live under the rule of vastly superior aliens. Several science fiction writers have actually done so, notably Arthur C. Clarke. But it’s not easy to do, and the resulting story won’t have nearly as many exciting battles as Independence Day.
UPDATE: Various commenters point to books like Niven and Pournelle’s Footfall, where the alien invaders got their technology for interstellar flight from others, and therefore aren’t much more advanced than humans in other ways. I am aware of several science fiction works along these lines. This is a possible solution to the screenwriter dilemma I described above. But notice that it requires exploring the aliens and their society in some detail (including their relationships with other alien races), which most Hollywood alien invasion movie producers don’t want to spend screen time on.