Fatal Flaws

While re-reading the latest Harry Potter (I'm so ashamed of how hooked I am on that series, especially considering how disappointing this latest offering was), I was spurred into one of my favorite rants about literature. Here goes. WHY is it that every villian, every evil plan must always have one fatal flaw? Sometimes it seems to be simply overlooked, which strikes me as silly when you've painted your villian as cunning and all-brilliant as you have. Take for example, the fourth (I think) Star Wars. Why, exactly, couldn't the Evil, almost Omnipotent Empire or Emperor think of running his own computer stimulation of the Death Star to see if maybe, just maybe, it had only one, ever so slight means of attack. I mean, why build long twisting tunnels just wide enough for a couple of planes leading to your one crucial weakness? Stupidity is the only reasonable explanation, and making your villians stupid makes your heroes a lot less impressive.
But even more annoying is when the one possible means of success seems to have been carefully planned by the creators of the system. Take, for example, Voldemort's defense of his Horcrux. It was in a cave that could be opened only by the offering of blood, then across a lake with a boat big enough for only one person (or at least, only one powerful wizard), and in a basin full of liquid that had to be drunk, but made the drinker unable to drain it. A brilliant defense! No, actually it was a stupid defense, as is evidenced by the fact that it failed not once, but twice. (To our knowledge. Who knows how many people were hopping in and out over the years). Because, of course, it had the fatal flaw that the wizard could bring a child or perhaps some other sentient creature along with him.
My question- Why the whole blinking system??! If we will go with the assumption that Voldemort wants to know if anyone is trying to get the Horcrux (not that he ever seemed to show up at the scene), why not have an alarm go off as soon as the blood is spilled, and the rock not open? Why have any sort of boat at all? Why not have a potion that knocks the drinker instantly unconscious, if you don't want him dead? Why put the stupid thing in the middle of the big basin in the middle of the island instead of hiding it or something? Why have the inferi set to go off after the Horcrux is already secured (and besides, won't they kill the person you seem so eager to keep alive)? So much foolishness.
Nor does it seem limited to the villians. Take, for example, the first book in the same series. Now it seems perfectly logical, but in the hindsight, the whole maze towards the Sorceror's Stone is ludicrous. If the point is to simply defend the blinking thing, why make it so that there is one key to the locked door? Why make there be any potion to get you through the walls of fire at all? (That actually seems like a good place to have the alarms go off and the authorities come get the bad guys as they sit trapped between the two walls of fire.) For that matter, if we can expect Rowling to use spells invented for later books, why not use a secret keeper and leave it in another place entirely? In short, why make it accessible at all?
And sadly enough, the answer always seems to be "Because the author needed it that way." I am sure that I would run into the same problems if I was ever constructing a complicated plot (a friend and I have a joke about letting the facts interfere with our plots), but nonetheless, it is very cheap. If a prize is found only at the end of a maze, there should be a darn good reason that it is there instead of being entirely inaccessible. The reason can be anything- your villian is a sadist, the whole thing is a test, the villian really didn't think of that idea, etc- but the reason ought not to be that it jsut fit your plot better that way.