Reading the 1e DMG description of how to create magic items, such as potions and scrolls, I am left with the strong impression that magic is Not As Easy As We Assume. I mean, according to Gygax's description, even the creation of the ink for a magical scroll requires some expremely rare and potent ingredients.
I think our minds tend to zoom into that scene of the Mage's Dusty Laboratory, with shelves lined with mason jars stuffed with magical ingredients. Ok, that's cool, I get it, but think back one step... How did all those cool magical ingredients get into those jars? We are talking some crazy stuff, like Blood of Dragon, Liver of Doppleganger, Pituitary Gland of Rakshasa, Testicle of Mountain Giant... Not exactly stuff lying around and about, if you see what I mean.
Someone, somewhere, had to go pull a Major Bad-ass Power Move, and forcibly take that Stinger off the tail of the Wyvern, that Heart had to be cut from behind the ribs of the Troll, those Lungs had to be removed from the chest of the Red Dragon, the Poison had to be mikled by hand from the Snakes of the Medusa Head, etc etc etc, AND get them back to the magic user, in short enough time that he could extract their Essense, and get it into that dusty-looking mason jar on that shelf over back there in the corner.
Obviously, a magic user is going to be willing to pay good money for these rare and difficult to find ingredients. Thus, we open a window into the workings of the Magical Economy.
I think we all tend to default into the Harry Potter school of magic, imagining that it is just something that happens with the wave of a wand and the right magical words. D&D magic is more like chemistry, harnessing the power of magical ingredients. It is hard work, it takes a lot of time, and it is therefore RARE, and EXPENSIVE.
The magical economy provides a never-ending basket of adventure hooks. Rather than just hanging out at taverns hoping to hear of something cool in a nearby woods, we would expect to find a well-established network of Wizard Want Ads.
Attaining supplies is also the exact plot device we need to coerce Magic Users into adventuring parties. Why else would these pencil neck weirdo geeks endanger their precious and vulnerable skins, taking time away from their all-important research and study, to confront deadly monsters? Trust me, these low STR, low CON types would much rather be surrounded by books than slinking into dungeons WITH NO ARMOR. Think about it, every spell they cast burns through more precious and expensive ingredients, and its a total pain in the ass to prepare properly in the first place.
Rich mages will happily pay people to go do the dangerous stuff and bring the precious ingredients to them. But what about a wizard who is down on his luck, running short on the gold? To him, accompanying a war party full of big-muscled meat-heads is just a bargaining chip to help him acquire more of his precious magical components.
Yes, he will agree to accompany your party on its (insane) quest, AND (perhaps) support it with some magical assistance (when absolutely needed), IF he gets time (and assistance) butchering the bodies of the slain magical creatures. THAT is probably the typical attitude of the "adventuring" magic user.
Oddly enough, it is the Mage who is most likely to be into the Hack and Slash campaigning mode. The fighters and thieves are surely most interested in acquiring All The Gold and Jewels with as Little Blood Spilled as Possible. XP for GP rewards their clever manueverings. Mages, on the other hand, NEED BODY PARTS!!!
You can easily see this conversation taking place:
Figher - "Great job, Thief, finding that secret entrance and hidden causeway. That led us right to the treasure!"
Thief - "Yeah, that idiot dragon surely don't need this huge pile of cash, he sure gonna be pissed when he comes back in to find it gonzo! haw haw"
Mage - "No, no, wait, guys, don't you think we should go back and take care of that evil beast, I mean, uh, think of the poor villagers, I really think we should kill it, and hack its body into small, easily-carried pieces...."
In a typical D&D society, how many people are even going to be attempting a career as a magic user? What, 1 in a 1000? In a decent-sized city of 10,000 people, that means there would only be 10 magic users in the whole city. How many of them would be a high enough level to even be able to create magic items??? Maybe 2, or 3? Think about it, only 2 or 3 guys able to create magic items for a WHOLE TOWN.
And on top of that, a potential magic item has to be crafted of only the finest, rarest, and most expensive material possible. If it is not a perfect physical specimen, the magic enchantment will not hold. Can't you just see this conversation happening with an ambitious young fighter:
"You want me to enchant THAT? Look, sonny, let me give you a clue. If you want a magic sword, you need to start with the finest dwarven-forged steel, crafted by their ancient master smithies, deep in the mountain under the earth, smelted from the purest alloys... NOT that half-bronze piece of junk hammered together last week in Billy's backyard furnace... Smack that up against a dragon's hide, it'll snap in like two second, kid..."
Ok, in summary, given 1) the need for only the finest (read: most expensive) raw material, 2) the scarcity of high level magic users doing the painstakingly slow enchanting, and 3) the perpetual shortage of magic ingredients taken from the dead bodies of powerful dangerous monsters....
Magic Will Be Rare. And Expensive.
But will give PC's endless adventuring hooks in their attempts to acquire those materials, and/or come to possess the magic items directly.