Why Can't Wizards Cast All Day?

This is a long-standing question of mine, as anybody who's ever happened to play D&D with me will tell you. But seriously, come on! Wizards, men and women who dedicate their lives to understanding the flow of magic, the occult formulae, can only cast a few spells a day. And then they're back to being the old dude with the nasty beard and a squiggly stick.

D&D has never properly explained this sort of thing, at least not to many peoples' satisfaction. Yeah, there's all sorts of grognards and neo-grognards pointing the way towards Jack Vance, and his mind-blasting spells that literally wrench themselves from your mind if you dare utilize the occult syllables.

But there's a thematic problem in that. It's hard to imagine something as mundane as "Read Magic", a spell which D&D tells us even the most lowly of scrubs can memorize from scratch, given training. You're telling me that spell, too, wrenches itself from the wizard's mind? Every time he casts it? It's not too much of a stretch to suppose that "Necratuul's Sanity-Blasting Chant of Hideous Doom" would wrench itself from your mind, but Magic Missile? Really? A spell that is only slightly better than a good crossbow bolt is worth that kind of treatment?

If it were up to me, I'd keep the spells-per-day, mind-wrenching approach for the more important spells, like Fireballs and Earthquakes and all the gigantic, flashy stuff. But there's got to be a divide, at least in my mind. Let the wizard cast his little scrub spells, maybe from a pool of points that refreshes itself every night. Let him cast Magic Missile, Read Magic, and all sorts of other moderately useful spells without any sort of preperation, or stress. And let the enormous, world-shaking spells be one per day, or even one per week.

Keeping the giant spells once per while would keep them interesting and special, and let wizards have room left over for parlor tricks, prestigidations, and other little silly things that amuse him and provide minor aid. But when it comes to the useful stuff, the mighty eldritch magics that astound the world, and smite his foes, make him pay for it a little. Maybe the spells cause damage when they're cast?