Remathilis
Legend
Okay, so the party sends in the member with the correct race to commission an item. The knowledge is there, there are just a few more barriers for the PCs to access it.
Assuming the PCs could find someone to make it. Elves might not sell their magical rings, and dwarves won't given dwarven weapons to humans (Think about how the elves in the Hobbit scoffed at Thorin carrying an elven blade). Or that elves only make X amount of rings per decade, and they are under strict tradition/punishment not to exceed that.
So someone had to make magic item on a regular basis as otherwise the god could not bless some of them or a fluke can't happen. Again, the knowledge is there.
But its incomplete knowledge: if you can't seal the deal, then all the knowledge you have is useless. These are less "why can't I make anything" and more "I want to make X item. Here is 50,000 gp. Yay!, I now have X" concerns.
And again, the knowledge is there, the price will only be higher. On the upside it is now rather easy to know where you likely will get certain magic item. Want a stave? Visit city near the valley.
If its even for sale. The only vorpal swords are those in the dungeons, unless you are willing to sell your soul for the knowledge to make them. Or that the rare wood from staves can only be harvested from trees once every ten years. Or that the Fromzan (who is currently exploring the Isle of Dread on the Elemental Plane of Water) can even be reached to be asked.
That is just another way of saying "Modern people simply can't do that stuff without any real reason. Deal with it". But as with all other points, the knowledge is there and a D&D adventuring career is usually short enough that such a magic item would last at least 10 levels if not more.
Its there because you, as the DM, says it is. I neglected option 11: That item doesn't exist and nobody can ever make it since I figured it was obvious.
Again, I don't care if crafting rules exist. *I* want control as to WHAT can be made and HOW its made. 5e gives me those options. 3e took them from me and said "Here you go players; you need X, Y and Z to make magic items and everything with a price tag is kosher!"
If a player wants to make a quest out of creating a Cloak of Displacement, then great. Adventures flow from that. I don't want "Ok, I spent 24 days, 12,000 gp, and 480 XP, here is my cloak. What will I make next?"