Which book should magic item creation be exclusively in?
I don't think there's any assumption there. 5e is designed around characters not having magical items.That's a tough one. I think Steeldragons is making a big assumption that items are necessarily treasure. If your basic game balance takes magic items into account, and if magic item creation is an expected part of the balance plan, then it ought to be in the PHB, because players are expected to do a lot of it. If, however, your typical adventuring spellcasters are designed to be quite happy going through their entire career without crafting anything, then go ahead and stick it in the DMG, with other largely optional rules.
I find the sentiment amusing that putting it in the DMG will somehow keep those rules out of players hands...