http://tvtropes.org/pmwiki/pmwiki.php/Main/LinearWarriorsQuadraticWizardsThere's no denying that full casters rock the house that is Dungeons & Dragons. Many people blame poor playtesting, flaws in the system, and the incredibly versatile customization options for casters.
These are all good reasons, but I think that there are other major factors, factors that go deeper than Dungeons & Dragons.
People expect mages to be awesome. The thieves and fighters are meant to be heroic everymen, capable of doing plausible stuff that we can imagine ourselves doing with enough training. Conan can nimbly dodge bad guys and lop off arms with a sword, and stuff like that has happened in the real world. Not so with magic.
Nobody blinks an eye in most settings when a mage calls forth a comet to crash upon the invading army or create a castle out of thin air. Magic is supposed to be this all-powerful force that goes beyond human limits. Mages are supposed to be
special, because having this power is a privilege that sets them apart. Even an impoverished hermit can have awesome power at his fingertips if he's a wizard.
There's also the factor of "realism" and "verisimilitude." It's much easier for us to believe that a mage can grow ten feet tall, but not as much for a barbarian (unless said barbarian is a shapechanging race or something). Why was Tome of Battle maligned by a vocal minority (Book of Weeaboo Fightan Magic!)? Because it lifted melee characters out of the "everyman hero" spot! How does a Warblade get super-sonar senses with the "Listening to the Air" ability? Sounds like magic!
Something I've also noticed is that power-ups to mages don't cause as much outrage when an equivalent thing is done to "everymen heroes." I remember reading about an Elfwood artists' D&D house rule: big creatures have big natural weapons, so they can get around a shield's defenses better due to sheer size. Apparently he instituted this rule because he got tired of the party fighter applying his shield bonus to AC against trolls or something.
On the surface, this house rule sounds plausible: a 90-foot giant stomping on a 6-foot fighter is going to connect unless the fighter can jump out of the way. In reality, this house rule screws the fighter even more. The poor guys' not doing as much damage as two-handed power attack or charging with a lance. Why punish him in such a cruel fashion? I also noticed that the guy didn't seem to have a problem with Wizards using their magic to negate attacks (He didn't bring this up). What about Mirror Image? Or Blur?
So why do we give Wizards Knock and Invisibility but nothing of equivalent power to the sneaky rogue? Because we have the idea that mages should be able to do everything and not be limited to class roles? The idea that magic is just plain better? I think the answer to these last two questions is "yes" in the minds of many.
Look at literature: In the 1,001 Arabian Nights, magicians can do all sorts of stuff the other heroes couldn't. They can fly, shapechange into ferocious beasts, lift a castle into the air, and turn into fire. But that was the point: In Middle Eastern folklore, magic was revealed to humanity by the angels Harut and Marut as a test of moral character. Said angels warned the humans that this was bad mojo. Those who learned magic got corrupted by the power and damned their souls.
The idea of the all-powerful mage in D&D is not just a product of poor game design; it's part of our cultural construct. When we make RPG games with elements based off of fantasy, mythology, and folklore, we bring those aspects in. And somewhere along the way Linear Warriors, Quadratic Wizards snuck in.