Because you have the DC vary from 5, 15 and 25 + item level. If the guy is making a 10th level item, does it matter if it's a one use item, a normal item of a set item? As you said, it just matters if he's 10th. And why would it be easier to find the same person if you want him to make a lower level item?
Technically I don't believe you can enchant a set item - I haven't looked it up, but item sets are supposed to be unique. I wouldn't allow it even if the book doesn't say one way or the other.
One use items are easier to create than full items - in fact, for a lot of them you just need to find an alchemist, not a full wizard. So I think it would be easier to find someone willing to create one than to find someone willing to create a full item.
If I'm looking for someone to make a level 10 item, why on earth would I ask to find an enchanter of higher than level 10? It just makes my DC's higher for no reason! Sure, the DM can say, "Good news! That high roll allows you to find a Level 15 enchanter, not just the Level 10 enchanter you're looking for!" But what good was that do me if I'm just going to have him create a level 10 item anyway?
(Ok, next time I want a Level 15 item, I can just say, "I'll go back to the Level 15 enchanter I already know!" At which point it's the DM's decision whether I can use the same resource or if he's unavailable and I need to roll again. But if I was just rolling "to find an item", and not searching specifically for an enchanter, then I could go back to the magic shop I found, or the thieves' guild, or whatever, so I still don't see a difference. This is all a guideline anyway - DM's are free to let players buy stuff automatically if they want.)
Enchanters do NOT equal Wizards. Why wouldn't there be blacksmiths, tradesmen AND enchanters? I can't see why there wouldn't be people that can do enchantments that go into business instead of adventuring. Much safer and easy work than robbing dragons for a living. you just make your EXP from skill challenges instead of fighting.
Why do you think all people that can enchant are world hopping, busy wizards? Heck, people have no problem finding people to rasie them from the dead do they? That's a ritual too isn't it?
Because I like my magicians to have some sense of mystery to them, even in a high-fantasy world. I specifically don't want magic users to be commodities you can count on. Raise Dead is a special case because being unable to raise your character from the dead brings the whole game to a halt.
This is the kind of thing that needs to be put to a vote and get the input of all the players - how common are NPC magic users in this setting, and can you expect to be able to hire them out for a fee?
Clearly this is a matter of taste, and your taste is on the other end of the spectrum from mine. However, I do think you're approaching this from the wrong angle in another sense. You're trying to work out how common it would be to hire an enchanter, and from that work out the DC to create magic items, and using "common sense" rules like, "Once you've found an enchanter, he'll do anything you want, so all magic items should be equal." But this is not the 4E way. The 4E way is, "Figure out how easily you want your players to be able to get magic items, and set the DC based on that. Then construct a narrative explaining what happens when they hit or miss the DC." I think the DC should be different for consumables and full magic items because this is
useful, and then the game world should be constructed around this. You say that the DC's should be equal because all items take the same effort to create - I say that consumables
must be easier to create, because the DC's are not equal!