Why can't a PC create an Artifact? Yes, I know, the Powers create a lot of 'em, but at least some are made by mighty Clerics and Wizards, and it seems to me that if these NPCs can do it, PCs should be allowed to try, too! Creating an Artifact should be very, very difficult, and it may have unintended special features and/or curses when completed, but it should not be flat-out impossible in my opinion! Minor Artifacts would be (slightly) easier to create... I don't like any rule that says so-and-so the Mighty Wizard can churn out Artifacts by the dozens, but PCs with identical statistics cannot! Yeah, Artifacts can be headaches if not properly handled, but if NPCs are allowed to create them... Unless, of course, you rule that NO ONE in the PRESENT DAY of the campaign (PC or NPC) can make them! (Usually, but not always, Artifacts were created in the distant past, but there are some noteable exceptions...)
PCs creating Artifacts...
I don't know, I think a wish spell from a powerful wizard could theoretically create an artifact. Instead of having an experience cost as such, the process would set your experience level to 0 and kill you with no hope of resurrection.
The closest I've ever come to allowing a PC to make an artifact was when a group of PCs once comitted certain actions that led to the implosion of a Demi-plane. They managed to escape by breaching it's boundary as it collapsed and spilling themselves out into the Deep Ethereal. I thought that would pretty much be it, but one PC decided he was going to try to find a remnant of the place and I thought it had too much potential to pass up. Basically, they found a glowy semi-substantial lump of goo that was all that was left of the fundamental concept of the plane. Course, they didn't know that. Yet. 
Led to lots of fun with accidental triggering of the artifact, temporal side effects, and a general fear of/inability to get rid of the blasted thing, regardless of it's potential power. Not to mention that PC being barred by Sigil's portals because Our Lady likes Sigil's physics just as they are.
But I'd never allow a PC to get away with intentionally creating something of Artifact power level. If I could be convinced, then it would be the crux of an entire campaign arc, and by no means a certainty that they would succeed. You don't just go out and craft a 'Staff of Shadows' or an 'Orb of Dragonkind'
I was thinking more the Twisted Body and Soul of [Your Name Here].
Basically yes. Artifacts are created in unique sets of circumstances. No one, not even the gods themeselves, can churn them out by the dozens. They require singular materials or events. They require substances that only existed at the dawn of time, secrets of spellcraft that no living souls know, certain conjunctions of the planes. It's fun to think of the exact requirements for various artifacts, though. What goes into the creation of an orb of annihilation and why is it so difficult?
It's dangerous to let PCs create artifacts because they're so powerful, but I don't see any absolute reason it couldn't happen once or twice in a campaign, if the PCs earn it and the power level of the game is sufficient to handle it. If they start mass-producing decks of many things or hands of Vecna, you're in trouble.
Hand of Vecna
Amount: 1 batch
Ingredients:
1 Vecna (suggested comatose)
1 sharp blade
3 scrolls of regeneration
1 ring of three wishes
1 escape plan and a good hiding spot
Directions:
Lop off one of Vecna's hands with the sharp blade. Cast regeneration on him to create another hand. While waiting for the new hand to grow, use one of your wishes to permanently preserve the dismembered hand with all of its power. Repeat until out of scrolls/wishes. Use escape plan and hiding spot before Vecna wakes up. Pray.
Related:
One can also create an equal amount of 'Eye of Vecna' from the same Vecna, but this requires double the wishes and a durable spoon. Only one 'Head of Vecna' can be created from any single Vecna in addition to the hand, as the regeneration would be wasted on the headless trunk. In such a case, however, the fun received would be increased exponentially.
Disclaimer:
Neither the author nor publisher of the above recipe is held liable for any damages, mutilation, death, multi-generational curses, or eternal imprisonment in Ravenloft that may be received after attempting any of the above. The quality of escape plan and hiding space are the sole responsibility of the user.
Ha ha! That's great, Iavas. "A durable spoon."
In the Forgotten Realms Campaign Setting the fifth chapter of the nether scrolls, called Ars Factum (Of the Creation of Artifacts), can teach the reader how to create artifacts.
-420
artifacts are like epic item creation feats, but i think that a non epic character ca do one whit a special event plus a very rare very espencive material and in a very rare circustance.
Who says they cannot?
If your the GM, go for it!
The ultimate fact-of-the-matter is that an Artifact has to be more than some mundane magical item which should require some epic feats, real questing, and a real need to exist.
The old Wrath of the Immortals boxed set indicated that one of the means a PC might try to prove he was worthy enough for his patron to open the door to immortality was to create such a thing.
Obviousy, it was intended that not just GMs need make such legendary items.
Yeah, I wasn't suggesting that PCs (or NPCs, for that matter) should be able to just churn out Artifacts whenever they want... I was thinking more along the lines of a one-time deal, at great cost to the character and requiring a dangerous quest for the required elements/ingredients... and of course, no guarantee of success! Then there's the fact that Artifacts tend to have undesireable drawbacks or curses... and some Artifacts are actually created by accident!
There are no rules for making them, because it would lead to powergamers cranking them out all the time. If you, as a DM, allow it under special circumstances its probably fine. That's just like inventing a revolutionary machine that increases the world's level of technological development (steam engine, or the like) or replacing the queen as a communist dictator. It might be good, but it is too special to follow a general set of rules, IMHO.
If PCs could make them, it would have to be an Epic level feat and/or require extraordinary circumstances.
Given the severe penalties that a PC suffers just for *using* an artifact, I can only imagine the penalties a PC would suffer for actually *creating* one. Mortals were not meant to wield such power. 
In other words, creation of an artifact might kill a player character, exile him to some remote demiplane, drain him of all his magical abilities, or something similar. Not necessarily a bad way to retire a PC, though.
Artifacts should definitely not be linked to the power of the characterm and even less to some kind of feat! We are beyond plain game mechanics here, and an artifact can´t just be annexed to a campaign, it has to be central. To my opinion, only extraordinary events could create an artifact, and no one should ever be allowed to decide creating one, but a power ready to part with a fraction of it´s own essence. Vecna didn´t decide to lose his hand or eye, The One and Only Ring was created by distorting a magic at the essence and origine of the races of middle earth, Excalibur was a godly present. You don´t go around and achieve some quest to create an artifact.. Well at least a "major" artifact. Nobody creates an artifact, the legend creates it. The important is not how poweful it is, but how it influences the whole game setting. And the curse associated with it is far worse than the thing itself, and ever, everlasting. An artifacts appears only when the situation gets out of control, and turns out mightily good or bad, but uncontrolled. It´s powers are not chosen by anyone, they just are. You can´t just go around and decide: ok, i´m gonna create this artifact that´ll have that and that power. They are not super magical item, they are part of the setting, and the center of a larger scheme. And don´t forget their effects are supposed to be much grander when used by or toward the people involved in their creation. This would give the pc a lot of importance, and must be very hard to handle and keep in control for the DM.
Artifacts need a history and legend associated with them, they need an unique set of circumstances before they become artifacts rather than items (or epic magic items).